Imaging medium

ABSTRACT

A process for producing an image uses an imaging medium comprising an acid-generating layer or phase comprising a mixture of a superacid precursor, a sensitizing dye and a secondary acid generator, and a color-change layer comprising an image dye. The sensitizing dye has first and second forms, the first form having substantially greater substantial absorption in a first wavelength range than the second form. The superacid precursor is capable of being decomposed to produce superacid by radiation in a second wavelength range, but is not, in the absence of the sensitizing dye, capable of being decomposed by radiation in the first wavelength range. The secondary acid generator is capable of acid-catalyzed thermal decomposition by unbuffered superacid to form a secondary acid. While at least part of the sensitizing dye is in its first form, the medium is imagewise exposed to radiation in the first wavelength range, thereby causing, in the exposed areas of the acid-generating layer, the formation of unbuffered superacid. The medium is then heated to cause, in the exposed areas, acid-catalyzed thermal decomposition of the secondary acid generator and formation of the secondary acid. The components of the acid-generating and color-change layers or phases are then mixed so that the secondary acid causes a change in color of the image dye, and the sensitizing dye is converted to its second form. The acid-generating layer or phase desirably includes a cosensitizer which is a reducing agent less basic than the secondary acid generator.

REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of our application Ser. No.08/232,725, filed Apr. 25, 1994 (now U.S. Pat. No. 5,441,850).

BACKGROUND OF THE INVENTION

This invention relates to an imaging medium and to a process forproducing an image.

Images can be generated by exposing a photosensitive medium to light inan imagewise fashion. Some conventional non-silver halide photosensitivecompositions contain molecules which are inherently photosensitive, sothat absorption of electromagnetic radiation brings about decompositionof, at most, as many molecules as photons absorbed. However, a dramaticincrease in the sensitivity of such photosensitive compositions can beachieved if the absorption of each photon generates a catalyst for asecondary reaction which is not radiation-dependent and which effectsconversion of a plurality of molecules for each photon absorbed. Forexample, systems are known in which the primary photochemical reactionproduces an acid, and this acid is employed catalytically to eliminateacid-labile groups in a secondary, radiation-independent reaction. Suchsystems may be used as photoresists: see, for example, U.S. Pat. Nos.3,932,514 and 3,915,706; and Ito et al., "Chemical Amplification in theDesign of Dry Developing Resist Materials", Polym. Sci. Eng., 23(18),1012 (1983).

Among the known acid-generating materials for use in this type ofprocess employing secondary, non-radiation dependent reactions arecertain diazonium, phosphonium, sulfonium and iodonium salts. Thesesalts, hereinafter referred to as superacid precursors, decompose toproduce superacids, i.e., acids with a pKa less than about 0, uponexposure to electromagnetic radiation. Other materials decompose toproduce superacids in a similar manner. However, in the absence of aspectral sensitizer, the known superacid precursors decompose to producesuperacid only upon exposure to wavelengths which the precursors absorb,which are typically in the short ultraviolet region (below about 280nm). The use of such wavelengths is often inconvenient, not leastbecause special optical systems must be used.

It is known that various dyes can sensitize the decomposition ofsuperacid precursors upon absorption by the dye of radiation which isnot significantly absorbed by the superacid precursor; see, for example,European Patent Application Publication No. 120,601. Unfortunately,however, due to the very low pKa of the superacid, many such dyes areprotonated by the superacid, so that no unbuffered superacid is produced(i.e., the sensitizing dye buffers any superacid produced). Since nounbuffered superacid is released into the medium, decomposition ofsuperacid precursors sensitized by these dyes cannot be used to triggerany secondary reaction which requires the presence of unbufferedsuperacid.

(The term "unbuffered superacid" is used herein to refer to superacidwhich is not buffered by the sensitizing dye, and which thus provides anacidic species stronger than that provided by the protonated sensitizingdye. Because of the extreme acidity of superacids and their consequenttendency to protonate even species which are not normally regarded asbasic, it is possible, and indeed likely, that "unbuffered superacid"will in fact be present as a species buffered by some component of theimaging medium less basic than the sensitizing dye. However, suchbuffering by other species may be ignored for the present purposes, solong as superacid is present as an acidic species stronger than thatprovided by superacid buffered by the sensitizing dye.)

Crivello and Lain, "Dye-Sensitized Photoinitiated CationicPolymerization", J. Polymer Sci., 16, 2441 (1978) and Ohe and Ichimura,"Positive-Working Photoresists Sensitive to Visible Light III,Poly(tetrahydropyranyl methacrylates) Activated by Dye-SensitizedDecomposition of Diphenyliodonium Salt", J. Imag. Sci., Technol., 37(3),250 (1993) describe small sub-groups of sensitizing dyes which aresufficiently non-basic that the buffered superacids produced can effectcertain acid-catalyzed reactions. However, the need to restrict thechoice both of sensitizers and of acid-catalyzed reactions may make itdifficult to design an efficient imaging system at a specific desiredwavelength.

A variety of non-basic, polycyclic aromatic compounds sensitizedecomposition of superacid precursors to produce unbuffered superacidupon exposure to longer wavelengths than the superacid precursors absorbthemselves. Such materials are discussed in, for example, DeVoe et al.,"Electron Transfer Sensitized Photolysis of 'Onium salts", Can. J.Chem., 66, 319 (1988); Saeva, U.S. Pat. No. 5,055,376; and Wallraft etal., "A Chemically Amplified Photoresist for Visible Laser Imaging", J.Imag. Sci. Technol., 36(5), 468-476 (1992).

U.S. application Ser. No. 08/141,852, filed Oct. 22, 1993 (now U.S. Pat.No. 5,453,345) and its parent, U.S. Pat. No. 5,286,612 (both assigned tothe same assignee as the present invention) describe a process by whicha wider variety of dyes than those discussed above may be used togetherwith a superacid precursor to generate free (unbuffered) superacid in amedium. In this process, acid is generated by exposing a mixture of asuperacid precursor and a dye to actinic radiation of a first wavelengthwhich does not, in the absence of the dye, cause decomposition of thesuperacid precursor to form the corresponding superacid, thereby causingabsorption of the actinic radiation and decomposition of part of thesuperacid precursor, with formation of a protonated product derived fromthe dye; then irradiating the mixture with actinic radiation of a secondwavelength, thereby causing decomposition of part of the remainingsuperacid precursor, with formation of free superacid. Generation ofsuperacid by exposure to the second wavelength may be sensitized by oneof the non-basic, polycyclic aromatic sensitizers mentioned above. (Forconvenience, the type of process disclosed in this patent willhereinafter be called the '612 process.)

However, the use of any of the methods described above for thesensitized decomposition of superacid precursors to generate unbufferedsuperacid poses a severe problem if the exposing wavelength for theimaging system falls within a wavelength range in which the resultantimage is to be viewed, and the sensitizing dye is not removed. Thisproblem will hereinafter be referred to as the "sensitizing/viewingproblem," and is particularly apparent when one attempts to use one ofthe sensitizers described above to make an image, intended to be viewedby the eye, by means of an exposure to visible wavelengths. Absorptionof visible light by the sensitizer leads to a large minimum opticaldensity (D_(min)) in the final image, lowering its contrast and makingits appearance unacceptable (especially in regions intended to be whim,that is diffusely reflective, and non-absorbing in the visible region).Even if the visible absorption of the sensitizer is bleached in thecourse of the sensitization reaction, this visible absorption will stillremain in the originally non-exposed areas of the image. Thesensitizing/viewing problem is not confined to visible wavelengths butapplies to any system in which the exposing radiation absorbed by thesensitizer falls within a wavelength range in which the resultant imageis to be viewed or used (in the case of a photomask or photographicnegative). For example, in the graphic arts industry, it is conventionalto expose contact and "dupe" films in the near ultra-violet to producephotomasks which display imagewise changes in absorption which arethemselves in the near ultra-violet range. Such processes, if sensitizedby one of the methods described above, may suffer from asensitizing/viewing problem.

The sensitizing/viewing problem is especially severe if it is desired toconstruct a full-color imaging system, which requires exposure to atleast three different wavelengths to produce images in three primarycolors. A conventional approach to this problem would be to resort to"false sensitization", i.e., to expose the imaging medium at threewavelengths which are not visible to the eye. In practice, however,exclusion of visible wavelengths for exposure leads to great difficulty.

Efficient sensitization of superacid precursors at non-visiblewavelengths is limited to the near ultra-violet region (between about330 nm and 420 nm) and the near infra-red region (between about 700 nmand 1200 nm). The non-basic, polycyclic aromatic compounds mentionedabove may be used to sensitize decomposition of superacid precursors tomake unbuffered superacid in the near ultra-violet region. However, itis difficult to find three near ultra-violet wavelengths which can begenerated by a convenient source and which are sufficiently separatedfrom each other to be absorbed by three sensitizers without cross-talkamong three color-forming layers. (The practical near ultra-violetwavelength range for this type of imaging process extends only fromabout 330 nm to about 420 nm, since other components of the imagingmedium, such as leuco image dyes if used, often absorb competitivelywith the sensitizer below about 330 nm, and wavelengths above about 420nm will appear yellow to the human eye). It is difficult to find a lampwhich emits three spaced wavelengths in the 330-420 nm range; aconventional mercury lamp emits only two usable wavelengths in thisrange. Moreover, a modulating device such as a liquid crystal cell (andpolarizers, if required) may be damaged by lengthy exposure to nearultra-violet radiation. Although phosphors emitting in the nearultra-violet are known, cathode ray tubes using such phosphors must becustom-made, and the emission spectra of such phosphors is often broad,leading to possible cross-talk. Another available exposure source, thenear ultra-violet laser, is expensive.

Although it may at first appear that sensitization at near infra-redwavelengths would solve the problems discussed above, infra-redsensitizers suffer from other problems. Some near infra-red dyes, andall non-basic polycyclic aromatic compounds which absorb in the nearinfra-red, have visible absorptions which contribute to the D_(min) ofthe image. Equally importantly, the photon energy in the near infra-redis less than about 40 kcal/mole, and in practice this low photon energyappears to limit the quantum efficiency at which the sensitizing dyecauses the decomposition of the superacid precursor. This limitedquantum efficiency limits the overall sensitivity of the process.

The problems discussed above are alleviated if at least one of theexposing wavelengths is in the visible region. If at least one exposingwavelength is in the visible or near infra-red region, a maximum of onlytwo exposing wavelengths must be found in the near ultra-violet.Furthermore, the higher photon energy available in the visible region,as compared with the near infra-red, is likely to lead to higher quantumefficiency, and therefore higher sensitivity, than can be expected fromnear infra-red exposure. There are also other reasons why exposure inthe visible region is preferred. Conventional cameras are designed touse visible light, and thus any medium intended to replace conventionalsilver halide film in cameras and produce a direct positive print ortransparency must use visible light to produce a visible image.Likewise, any medium intended to produce a print from a photographicnegative must be capable of exposure by visible light. Conventionalphotographic printers, also, are designed to use visible light. Mucheffort has been expended in developing sources (for example,light-emitting diodes, cathode ray tubes, lumps and lasers) and controldevices (for example, liquid crystal light valves) optimized for visibleradiation. Accordingly, using visible radiation rather than (say)infra-red or ultra-violet radiation in an imaging process often enablesthe cost of the imaging apparatus to be reduced, and may also make itpossible to use a standard, commercially available light source orcontrol device rather than a custom-made source or device.

U.S. Pat. No. 5,334,489 and U.S. application Ser. No. 08/141,860 (nowU.S. Pat. No. 5,395,736) (both of which are assigned to the sameassignee as the present application), and the correspondingInternational Application No. PCT/US93/10224, Publication No. WO94/10607), all describe processes for the photochemical generation ofacid and for imaging using conventional ultra-violet sensitizers; theseprocesses will hereinafter collectively be called the "'489 process."The aforementioned U.S. Pat. No. 5,286,612 and copending applicationSer. No. 08/141,852 (both of which are assigned to the same assignee asthe present application), and the corresponding InternationalApplication No. PCT/US93/10215 (Publication No. WO 94/10606), alldescribe an imaging process using an imaging medium comprising anacid-generating layer or phase and a color-change layer or phase. Inthis patent and these applications, the acid-generating layer or phasecomprises a mixture of a superacid precursor, a sensitizer and asecondary acid generator. The secondary acid generator is capable ofacid-catalyzed thermal decomposition by unbuffered superacid to form asecondary acid. The color-change layer comprises an image dye whichundergoes a change in its absorption of radiation upon contact with thesecondary acid. After imagewise exposure and generation of unbufferedsuperacid in the exposed areas, the medium is heated; in exposed areas,the unbuffered superacid causes acid-catalyzed decomposition of thesecondary acid generator, thereby causing the formation of a molaramount of secondary acid much larger than the molar amount of unbufferedsuperacid present before the heating. In the non-exposed areas, however,since no unbuffered superacid is present, no significant generation ofsecondary acid takes place during the heating. Thereafter, the medium isfurther heated (in practice the two heating steps can be combined) tocause the components present in the two layers to mix, so that, inexposed areas, the secondary acid brings about the absorption change inthe image dye, thereby forming an image. Thus, the imaging medium is asingle sheet which develops its image without any need for treatmentwith a developing composition and without requiring any waste materialto be peeled from the medium to produce the final image.

In these processes, the sensitizer or sensitizers and the undecomposedsuperacid precursor remain present in at least the non-exposed areas ofthe medium after imaging (these non-exposed areas are the D_(min) areas,i.e., the regions of minimum optical density, in the normal case wherethe image dye is colorless before imaging and develops color in theexposed areas during imaging). Accordingly, in practice it is necessarythat the image be viewed in a wavelength range which does not includethe wavelength used for the imagewise exposure, since otherwise theimage will suffer from a sensitizing/viewing problem, with the contrastbetween the exposed and non-exposed areas reduced by the absorption, inthe non-exposed areas, of the sensitizer or sensitizers used. Forexample, as discussed above, the preferred '612 process requires twoexposures, one at a wavelength in the near infra-red region (700-1200)nm and the second at a wavelength in the near ultra-violet region,typically around 365 nm, to produce an image in the visible region(about 400-700 nm).

There is a need for a modified form of these imaging media and processeswhich removes the restriction that the wavelength used for the imagewiseexposure be outside the wavelength range in which the final image isviewed.

SUMMARY OF THE INVENTION

This invention provides a process for producing an image. This processuses an imaging medium comprising an acid-generating layer or phasecomprising a mixture of a superacid precursor, a sensitizing dye and asecondary acid generator, and a color-change layer or phase comprisingan image dye. The sensitizing dye has a first form and a second form,the first form having substantially greater absorption in a firstwavelength range than the second form. The superacid precursor iscapable of being decomposed (if necessary, with the addition of anon-basic sensitizer, for example a polycyclic aromatic sensitizer) toproduce superacid by actinic radiation in a second wavelength rangedifferent from the first wavelength range, but is not, in the absence ofthe sensitizing dye, capable of being decomposed to produce superacid byactinic radiation in the first wavelength range. The secondary acidgenerator is capable of acid-catalyzed thermal decomposition byunbuffered superacid to form a secondary acid. The image dye undergoes achange in its absorption of radiation upon contact with the secondaryacid. In the present process, while at least part of the sensitizing dyeis in its first form, the medium is imagewise exposed to actinicradiation in the first wavelength range. In the exposed areas of theacid-generating layer or phase, this imagewise exposure causes thesensitizing dye to decompose at least part of the superacid precursor,with formation of unbuffered superacid. Thereafter, the medium is heatedto cause, in the exposed areas of the acid-generating layer or phase,acid-catalyzed thermal decomposition of the secondary acid generator andformation of the secondary acid. Next, the components of theacid-generating and color-change layers or phases are mixed, therebycausing, in the exposed areas of the medium, the secondary acid to bringabout the change in absorption of the image dye and thereby form theimage. Finally, in at least the non-exposed areas of the medium, thesensitizing dye is converted to its second form.

This invention also provides an imaging medium as defined in thepreceding paragraph.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D show the acid concentrations in the exposed and non-exposedregions of the acid-generating layer during the various steps of a firstpreferred process of the present invention;

FIGS. 2A-2E show the acid concentrations in the exposed and non-exposedregions of the acid-generating layer during the various steps of asecond preferred process of the present invention; and

FIG. 3 is a schematic cross section through an imaging medium of thepresent invention as it is being passed between a pair of hot rollersduring the imaging process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention differs from the '489 and '612processes in that the present process uses a sensitizing dye having afirst form and a second form, with the first form having substantiallygreater absorption in a specific wavelength range (herein called the"first wavelength range") than the second form. The medium is imagewiseexposed to radiation in the first wavelength range while the sensitizingdye is in its first form, so that, in the exposed areas, the sensitizingdye causes decomposition of at least part of the superacid precursor,with formation of unbuffered superacid. As explained in more detailbelow, in some cases, in addition to the imagewise exposure, furthersteps may be necessary to produce the imagewise distribution ofunbuffered superacid required for later steps of the present process.

The unbuffered superacid in the exposed areas (in effect, a "latentimage" in superacid) produced in the first part of the present processis then used to cause superacid-catalyzed thermal decomposition of thesecondary acid generator, and for this purpose the imaging medium isheated. Next, the components of the acid-generating and color-changelayers or phases are mixed, thereby causing, in the exposed areas of themedium, the secondary acid to bring about the change in absorption ofthe image dye and to form the image. (In saying that the components ofthe acid-generating and color-change layers or phases are mixed, we donot exclude the possibility that these two layers or phases may bemerged to form a single layer or phase, but such complete merger is notnecessary, since it is only necessary for the secondary acid and theimage dye to come into contact, so that the secondary acid can cause thechange in absorption of the image dye.) Finally, in at least thenon-exposed areas of the imaging medium, the sensitizing dye isconverted to its second form. This conversion of the sensitizing dye toits second form essentially removes from the non-exposed areas of theimage the absorption of the sensitizing dye in the first wavelengthrange, and thus allows the imagewise exposure of the medium to beeffected using radiation in the same wavelength range as that in whichthe image is viewed; at least the non-exposed areas of the final imagedo not absorb at the wavelength at which imaging is effected.

Conversion of the sensitizing dye from its first to its second form maybe effected by any technique capable of convening a dye from a firstform having a substantial absorption in the desired wavelength range toa second form having substantially less absorption in this wavelengthrange. However, this conversion should be such as not to adverselyaffect the image, either by significantly decreasing the maximum opticaldensity (D_(max)) of the exposed regions or by introducing unwantedcolor into the regions where the sensitizing dye finally remains in itssecond form. In addition, of course, the sensitizing dye should bechosen so that it is compatible with the other components of the imagingmedium and does not, for example, crystallize out on prolonged storageor undergo slow thermal reactions with other components of the imagingmedium during such storage.

The conversion of the sensitizing dye from its first form to its secondform may be either a reversible or an irreversible chemical reaction.(In theory, no chemical reaction is thermodynamically completelyirreversible; however, as any chemist is aware, there are many reactionswhere the equilibrium lies so far to one side that no detectable traceof the other supposed components of the equilibrium mixture are present,and the term "irreversible" is used herein to mean such reactions whichare for practical purposes irreversible.) For example, conversion of thefirst form of the sensitizing dye to the second form may be effected bycontacting the sensitizing dye with a base; in a preferred variant ofthe present process of this type (hereinafter called the "deprotonationprocess"), the first form of the sensitizing dye is a protonated formand the second form is a deprotonated form, and the two forms arereversibly interconverted by contact with base or acid. Alternatively,conversion of the first form of the sensitizing dye to the second formmay be effected by reacting the sensitizing dye with a nucleophile; thisvariant of the present process is hereinafter called the "nucleophileprocess". In such a process, the chemical change effected by thenucleophile may be irreversible. Other techniques for converting a firstform of the sensitizing dye to its second form may also be used, forexample heating the imaging medium to cause thermal decomposition of thesensitizing dye, or bringing about decomposition of the sensitizing dyeby exposing the imaging medium to actinic radiation of a wavelengthwhich does not affect the other components of the medium.

The imaging media of the invention described thus far still contain,after imaging, a substantial amount of unchanged superacid precursor,and are thus susceptible to post-imaging color changes caused byunwanted generation of superacid by ambient radiation striking thesuperacid precursor, with consequent generation of acid. However, thissusceptibility of the imaged media to unwanted color generation can beeliminated by including in the media a "fixing" reagent capable ofdestroying the superacid precursor without formation of acid therefrom,thus fixing the image. Such fixing reagents, and processes for theiruse, are described and claimed in copending application Ser. No.08/232,757, and its continuation-in-part application Ser. No.08/430,421, of even date herewith and assigned to the same assignee asthe present application. The preferred embodiments of the inventiondescribed below with reference to Tables 1 and 2 and the accompanyingdrawings contain such a fixing reagent.

In the present process, the imagewise exposure is effected with at leastpart of the sensitizing dye in its first form. The imaging medium may beprepared with at least part of the sensitizing dye already its firstform; for example in a deprotonation process, an appropriate amount ofacid may be included in a coating solution from which theacid-generating layer is deposited, or this coating solution may becontacted with a separate phase containing an acid. Alternatively, theimaging medium may be prepared with the sensitizing dye in its second orsome other precursor form, and converted to its first form within theacid-generating layer before the imagewise exposure is effected; forexample, as illustrated below with reference to FIGS. 1A-1D and 2A-2E,in a deprotonation process, the imaging medium may be prepared with thesensitizing dye in its deprotonated (second) form and this deprotonatedform converted to the protonated (first) form prior to the imagewiseexposure by generating or introducing acid into the acid-generatinglayer. Such acid generation is conveniently effected by exposing thewhole of the acid-generating layer to radiation of the secondwavelength, with consequent formation of superacid from the superacidprecursor, and protonation of at least part of the sensitizing dye. Acombination of these two methods of providing acid may of course beused, i.e., one could coat a limited amount of sensitizing dye in itsfirst form and generate additional sensitizing dye in its first formimmediately before use.

To illustrate the complex chemical reactions which may take place duringthe present process, two preferred deprotonation processes of thepresent invention will now be described, with reference to Tables 1 and2 below and FIGS. 1A-1D and 2A-2E of the accompanying drawings.

Table 1 and FIGS. 1A-1D of the accompanying drawings show the changes inacid concentration in exposed and non-exposed areas of theacid-generating layer used at various stages during the first preferreddeprotonation process. The last section of Table 1, headed "AFTERFIXING," shows the composition of the combined acid-generating andcolor-change layers after the components thereof have become intermixed.

                                      TABLE 1                                     __________________________________________________________________________    EXPOSED AREA        NON-EXPOSED AREA                                          Component       Moles                                                                             Component     Moles                                       __________________________________________________________________________    PRIOR TO USE                                                                  [S-DYE]         1   [S-DYE]       1                                           Secondary acid generator                                                                      10  Secondary acid generator                                                                    10                                          Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                              5   Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                            5                                           AFTER INITIAL ULTRA-VIOLET EXPOSURE                                           [S-DYE-H].sup.+ SbF.sub.6.sup.-                                                               0.75                                                                              [S-DYE-H].sup.+ SbF.sub.6.sup.-                                                             0.75                                        [S-DYE]         0.25                                                                              [S-DYE]       0.25                                        Secondary acid generator                                                                      10  Secondary acid generator                                                                    10                                          Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                              4.25                                                                              Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                            4.25                                        AFTER IMAGEWISE VISIBLE EXPOSURE                                              [S-DYE-H].sup.+ SbF.sub.6.sup.-                                                               0.25                                                                              [S-DYE-H].sup.+ SbF.sub.6.sup.-                                                             0.75                                        Ph-[S-DYE-H].sup.+ SbF.sub.6.sup.-                                                            0.75                                                                              [S-DYE]       0.25                                        HSbF.sub.6      0.5 Secondary acid generator                                                                    10                                          Secondary acid generator                                                                      10  Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                            4.25                                        Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                              3.5                                                           AFTER HEATING                                                                 [S-DYE-H].sup.+ SbF.sub.6.sup.-                                                               0.25                                                                              [S-DYE-H].sup.+ SbF.sub.6.sup.-                                                             0.75                                        Ph-[S-DYE-H].sup.+ SbF6-                                                                      0.75                                                                              [S-DYE]       0.25                                        HSbF.sub.6      0.5 Secondary acid generator                                                                    10                                          Secondary acid  10  Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                            4.25                                        Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                              3.5                                                           AFTER FIXING                                                                  [S-DYE]         0.25                                                                              [S-DYE]       1                                           Ph-[S-DYE]      0.75                                                                              HOAc          0.75                                        HOAc            6   KOAc          1                                           Image dye/secondary acid salt                                                                 5.5 Secondary acid generator                                                                    10                                          Unprotonated image dye                                                                        1   Cu reagent    1                                           Ph-image dye/secondary acid                                                                   3.5 PhOAc         4.25                                        salt                Unprotonated image dye                                                                      10                                          Cu reagent      1   KSbF.sub.6    5                                           KSbF.sub.6      5                                                             K/secondary acid salt                                                                         1                                                             __________________________________________________________________________

As shown in Table 1, the imaging medium initially contains thesensitizing dye in its unprotonated form. Both the exposed andnon-exposed areas comprise a quantity (shown in Table 1 as 1 mole forsimplicity; all references to moles concerning Tables 1 and 2 (seebelow) refer to moles per unit area of the imaging medium, and are onlyby way of illustration, since the proportions of the various componentsmay vary widely) of a sensitizing dye, a larger molar quantity of asuperacid precursor (5 moles of Ph₂ I⁺ SbF₆ ⁻ are shown in Table 1; asuitable quantity of a non-basic polycyclic aromatic sensitizer, such aspyrene, is also included in the medium but is not shown in Table 1) anda still larger molar quantity (10 moles are shown in Table 1) of asecondary acid generator.

The imaging medium is first blanket irradiated with radiation in thesecond wavelength range, typically near ultra-violet radiation, theamount of radiation applied being sufficient to cause the decompositionof less than one mole (0.75 mole is used for illustration in Table 1 andFIG. 1A) of the superacid precursor, thus producing a correspondingamount of superacid. This superacid immediately protonates thesensitizing dye, producing a salt of the dye shown as "[S-DYE-H]⁺ SbF₆ ⁻" in Table 1, and leaving no unbuffered superacid present in theacid-generating layer. Thus, after this initial ultra-violet exposure,as shown in Table 1, all areas of the acid-generating layer contain 0.75mole of the sensitizing dye salt, 0.25 mole of unprotonated sensitizingdye, 4.25 moles of superacid precursor and 10 moles of secondary acidgenerator. This situation is illustrated in FIG. 1A, which shows theacid level as 0.75 times a threshold level (indicated by T in FIGS.1A-1D) at which all the sensitizing dye becomes protonated.

(The secondary reactions that follow the fragmentation of the superacidprecursor are not entirely understood at present. However, it is likelythat a phenyl radical is generated, which subsequently becomes attachedto the radical cation derived from the non-basic polycyclic aromaticsensitizer, following which elimination of a proton occurs. This phenylradical is ignored in Table 1. Even if some of the phenyl radicalsgenerated do become attached to sensitizing dye molecules, this will notsignificantly affect the overall course of the process shown in Table 1and FIG. 1, since a phenylated form of the sensitizing dye would beexpected to undergo the same type of protonation and deprotonationreactions, with similar absorption shifts, as the non-phenylated dye.)

After the initial ultra-violet exposure, the imaging medium is imagewiseexposed to radiation in the first wavelength range; visible radiation isshown for illustration in Table 1. As shown in Table 1 and FIG. 1 B, inthe area BC of the acid-generating layer which is exposed to the visibleradiation, this visible radiation causes the protonated sensitizing dyeto bring about the decomposition of a further 0.75 mole of superacidprecursor, with generation of a further 0.75 mole of superacid, so thatthe total amount of acid present exceeds the threshold T. The additionalsuperacid generated by the visible exposure protonates the remaining0.25 mole of previously unprotonated sensitizing dye, leaving 0.5 moleof unbuffered superacid in the exposed area BC, as shown in FIG. 1B.(For purposes of illustration, FIG. 1B shows the acid generated in theultraviolet and visible exposures separately, although of course nodifference exists chemically.) In the non-exposed areas AB and CD nochange in the acid level occurs, the acid concentration remains belowthe threshold T, and no unbuffered superacid is present after thevisible exposure.

Thus, at the end of the imagewise irradiation, unbuffered superacid ispresent in the exposed areas, whereas in the non-exposed areas nounbuffered superacid is present, all the superacid generated beingbuffered by the sensitizing dye. In effect, the acid-generating layernow contains a "latent image" in superacid, although this image is notvisible to the human eye.

It is expected that the decomposition of the superacid precursor by thesensitizing dye during the imagewise visible exposure will beaccompanied by phenylation of the photooxidized sensitizing dye by thephenyl radical derived from the superacid precursor, followed byelimination of a proton. Accordingly, at the end of the imagewiseexposure, the exposed areas will contain 0.75 mole of a phenylatedproduct derived from the protonated sensitizing dye, this product beingdenoted Ph-[S-DYE-H]⁺ SbF₆ ⁻ in Table 1. The remaining 0.25 mole ofsensitizing dye will remain in the [S-DYE-H]⁺ SbF₆ ⁻ form. Also presentin the exposed areas will be 0.5 mole of unbuffered superacid, the 3.5remaining moles of superacid precursor, and the 10 moles of secondaryacid generator, which remain unchanged at this point. (The compositionof the non-exposed areas of course remains unchanged by the imagewisevisible exposure.)

The imaging medium is next heated. In the exposed area BC, theunbuffered superacid present catalyzes the decomposition of thesecondary acid generator, thus producing a large quantity of thesecondary acid (10 moles are shown by way of example in Table 1; FIG. 1Cis not strictly to scale). However, in the non-exposed areas AB and CD,no unbuffered superacid is present, and the sensitizing dye/superacidsalt does not catalyze the decomposition of the secondary acidgenerator, so that essentially no decomposition of the secondary acidgenerator occurs and essentially no secondary acid is generated.

In the final step of the process, as discussed in more detail below, thecomponents of the acid-generating and color change layers becomeintermixed. Table 1 assumes that the color-change layer contains 10moles of an indicator image dye, 1 mole of copper compound, 1 mole of areducing agent (the products produced by oxidation of this reducingagent are ignored in Table 1 for simplicity) and 6 moles of a reactivematerial, shown as potassium acetate in Table 1 (where acetate isabbreviated "OAc"). Table 1 further assumes that the image dye is morebasic than the sensitizing dye; the contrary case is discussed belowfollowing Table 2. In the non-exposed areas, the copper compound, thereducing agent and the reactive material decompose all remainingsuperacid precursor, with generation of the corresponding amount ofphenyl acetate (Table 1 assumes), phenyl iodide (omitted from Table 1)and potassium hexafluoroantimonate. In the exposed areas, the potassiumacetate is protonated by the superacid and by some of the secondaryacid. The copper reagent catalyzes decomposition of the remainingsuperacid precursor with the formation of phenyl cations, which reactwith the most nucleophilic species remaining, here assumed to be theimage dye. (In practice, the decomposition of the superacid precursor isprobably somewhat more complicated, and other products may be produced;however, the exact type of decomposition products produced does notaffect the essential nature of the present process.)

In the exposed areas, the unbuffered superacid and 4.5 moles of thesecondary acid are immediately neutralized by the potassium acetate,which also deprotonates the protonated forms of both the originalsensitizing dye and the phenylated form of this dye to produce thecorresponding unprotonated dyes, thereby removing the absorption in thefirst wavelength range due to the sensitizing dye. The decomposition ofthe superacid precursor is catalyzed by the copper compound, leading tothe formation of 3.5 moles of phenylated image dye. 5.5 Moles of thesecondary acid reversibly protonate and form a salt with the image dye.Both the phenylated and the protonated image dyes are colored. 1 Mole ofthe image dye remains in its unprotonated, leuco form. FIG. 1D showsgraphically the 5.5 moles of secondary acid (3.5 moles of aryl cationare also formed) remaining in the exposed areas.

In the non-exposed areas, the potassium acetate deprotonates thesensitizing dye, returning it to its unprotonated form, and thusreducing the D_(min) of the image in this range (assuming, as is usual,that the absorption change in the image dye is an increase inabsorption, i.e., increase in color, in the relevant wavelength range sothat the non-exposed areas are the D_(min) areas of the image). Thedecomposition of the superacid precursor and the deprotonation of thesensitizing dye consumes 5 moles of potassium acetate; 1 mole ofpotassium acetate remains in the non-exposed areas. This excess of baseis represented in FIG. 1D as -1 moles of remaining acid. None of theimage dye is protonated, all remaining in its unprotonated, leuco form.The provision of the excess potassium acetate serves to ensure that, ifa small amount of uncatalyzed thermal decomposition of the secondaryacid generator does occur in non-exposed areas AB and CD during theheating step, the small amount of secondary acid resulting will beneutralized by base before the secondary acid can effect changes in theimage dye, as described in more detail below. The excess potassiumacetate also ensures that, if minor decomposition of the secondary acidgenerator does occur after the imaging process has been completed, theminor amounts of acid generated will be neutralized by the potassiumacetate and thus will not affect image dye in the non-exposed areas ofthe final image.

It will be seen from Table 1 that the "neutralization" of the superacidand secondary acid, and deprotonation of the protonated sensitizing dyeby the potassium acetate, produce acetic acid. Although acetic acid isnormally regarded as an acid, it is insufficiently acidic in thepolymeric binders in which the present process is normally carded out toprotonate the sensitizing dye or the image dye, and is thus not regardedas an acid for present purposes.

From the foregoing description, it will be seen that, in the exposedareas, the superacid catalyzes the breakdown of the secondary acidgenerator, so that the final quantity of secondary acid present issubstantially larger than the quantity of unbuffered superacid produceddirectly by the imagewise radiation acting on the superacid precursor,although of course the secondary acid is typically a weaker acid thanthe superacid itself. This "chemical amplification" of the superacid bythe secondary acid generator increases the number of moles of acidgenerated per einstein of radiation absorbed, and thus increases thecontrast of the image produced by the present process as compared withsimple generation of superacid by a superacid precursor. In practice, ithas been found that, under proper conditions, at least 20, and in somecases 100 or more, moles of secondary acid can be liberated for eachmole of unbuffered superacid present in the exposed areas following theimagewise irradiation.

The first preferred deprotonation process described above is convenientfor use with many combinations of indicator sensitizing dye andsecondary acid generator. However, this first preferred process requiresthat the superacid generated during the imagewise exposure effectacid-catalyzed thermal decomposition of the secondary acid generator inthe presence of the protonated sensitizing dye, and such acid-catalyzeddecomposition requires that the superacid protonate the secondary acidgenerator. If the sensitizing dye is capable of further protonation, andthe singly protonated sensitizing dye is significantly more basic thanthe secondary acid generator, any superacid generated above thethreshold T during the imagewise exposure will tend to doubly protonatethe sensitizing dye rather than protonate the secondary acid generator.If the doubly protonated sensitizing dye absorbs the wavelength used forthe imagewise exposure and also causes decomposition of the superacidprecursor, such double protonation of the sensitizing dye is not aserious problem; the imagewise exposure can be continued until theconcentration of acid in the exposed areas exceeds 2T, at which pointall the sensitizing dye is doubly protonated, and unbuffered superacidis again available to protonate the secondary acid generator. After thiscontinued exposure, the acid level diagram is similar to that shown inFIG. 1B, except that the acid concentration in the exposed area BCexceeds 2T rather than T. The subsequent heating and base addition stepscan be carried out exactly as described above with reference to FIGS. 1Cand 1D.

If, however, the sensitizing dye can be doubly protonated, the singlyprotonated sensitizing dye is significantly more basic than thesecondary acid generator, and the doubly protonated form of thesensitizing dye either does not absorb the wavelength used for theimagewise exposure, or does absorb this wavelength but does not causedecomposition of the superacid precursor following such absorption,continuing the imagewise exposure will result in the acid concentrationincreasing to, at most, 2T, at which point all the sensitizing dye isdoubly protonated, and further acid generation ceases. The presentprocess can be carried out with this type of sensitizing dye, butrequires an additional step, as will now be described with reference toTable 2 and FIGS. 2A-2E. In view of the number of possible species inthe second preferred process illustrated in Table 2 and FIGS. 2A 2E andthe uncertainties as to, for example, the exact species which isphenylated by the phenyl radicals liberated upon decomposition of thesuperacid precursor, or which counterion or mixture of counterions isassociated with a given protonated sensitizing dye species, forsimplicity the various possible protonated species derived from thesensitizing dye are grouped together without regard to their degree ofphenylation. Thus, for example the term "doubly protonated S-DYEspecies" used in Table 2 refers to all doubly protonated species derivedfrom the sensitizing dye, whether those species are substituted with 0,1 or 2 phenyl groups derived from the superacid precursor.

                                      TABLE 2                                     __________________________________________________________________________    EXPOSED AREA         NON-EXPOSED AREA                                         Component        Moles                                                                             Component        Moles                                   __________________________________________________________________________    PRIOR TO USE                                                                  [S-DYE]          1   [S-DYE]          1                                       Secondary acid generator                                                                       10  Secondary acid generator                                                                       10                                      Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                               5   Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                               5                                       AFTER INITIAL ULTRA-VIOLET EXPOSURE                                           Singly protonated S-DYE species                                                                0.75                                                                              Singly protonated S-DYE species                                                                0.75                                    SbF.sub.6.sup.-  0.75                                                                              SbF.sub.6.sup.-  0.75                                    [S-DYE]          0.25                                                                              [S-DYE]          0.25                                    Secondary acid generator                                                                       10  Secondary acid generator                                                                       10                                      Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                               4.25                                                                              Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                               4.25                                    AFTER IMAGEWISE VISIBLE EXPOSURE                                              Doubly protonated S-DYE species                                                                0.75                                                                              Singly protonated S-DYE species                                                                0.75                                    Singly protonated S-DYE species                                                                0.25                                                                              SbF.sub.6.sup.-  0.75                                    SbF.sub.6.sup.-  1.75                                                                              [S-DYE]          0.25                                    Secondary acid generator                                                                       10  Secondary acid generator                                                                       10                                      Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                               3.25                                                                              Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                               4.25                                    AFTER SECOND ULTRA-VIOLET EXPOSURE                                            Doubly protonated S-DYE species                                                                1   Doubly protonated S-DYE species                                                                0.75                                    SbF.sub.6.sup.-  2   Singly protonated S-DYE species                                                                0.25                                    Secondary acid generator                                                                       10  SbF.sub.6.sup.-  1.75                                    HSbF.sub.6       0.75                                                                              Secondary acid generator                                                                       10                                      Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                               2.25                                                                              Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                               3.25                                    AFTER HEATING                                                                 Doubly protonated S-DYE species                                                                1   Doubly protonated S-DYE species                                                                0.75                                    SbF.sub.6.sup.-  2   Singly protonated S-DYE species                                                                0.25                                    HSbF.sub.6       0.75                                                                              SbF.sub.6.sup.-  1.75                                    Secondary acid   10  Secondary acid generator                                                                       10                                      Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                               2.25                                                                              Ph.sub.2 I.sup.+ SbF.sub.6.sup.-                                                               3.25                                    AFTER FIXING                                                                  Unprotonated S-DYE species                                                                     1   Unprotonated S-DYE species                                                                     1                                       HOAc             6   HOAc             0.75                                    Image dye/secondary acid salt                                                                  6.75                                                                              Unprotonated Image dye                                                                         10                                      Unprotonated image dye                                                                         1   KOAc             1                                       Ph-image dye/secondary acid salt                                                               2.25                                                                              Secondary acid generator                                                                       10                                      Cu reagent       1   Cu reagent       1                                       KSBF.sub.6       5   KSBF.sub.6       5                                       K/secondary acid salt                                                                          1   PhOAc            3.25                                    __________________________________________________________________________

The second preferred deprotonation process of the invention shown inTable 2 begins in exactly the same manner as the first preferreddeprotonation process described above with reference to Table 1; thesame amounts of the same components are initially present in the imagingmedium, and the same initial ultraviolet exposure is effected togenerate 0.75 mole of buffered superacid throughout the imaging medium,thus converting 0.75 mole of sensitizing dye to its protonated form.Accordingly, FIG. 2A is essentially identical to FIG. 1A. However, inthe next step, namely the imagewise exposure, the exposure is preferablycontinued until a further 1 mole of superacid precursor is decomposed bythe sensitizing dye, so that 1.75 moles of acid are present in theacid-generating layer, as shown in FIG. 2B. If the sensitizing dye is ofthe type which is "consumed" by reaction with one molecule of superacidprecursor, so that each mole of sensitizing dye can only generate onemole of superacid, the acid-generating reaction is self-limiting at thispoint. It is not essential that the imagewise exposure be continued tothis point, since the only requirement for the later stages of theprocess is that, after the imagewise exposure, there be a substantialdifference in acid concentration between the exposed and non-exposedareas. Accordingly, it may be convenient to continue the imagewiseexposure to this point, thus rendering the imaging process lesssusceptible to temporal or areal variations in the radiation source usedfor the imagewise exposure. As shown in Table 2, at the conclusion ofthe imagewise exposure, the exposed areas of the acid-generating layercontain 0.75 mole of doubly protonated sensitizing dye species, 0.25mole of singly protonated sensitizing dye species, 10 (unchanged) molesof secondary acid generator, and 3.25 moles of superacid precursor. (Asin the first preferred process, obviously the imagewise exposure causesno change in the composition of the non-exposed areas of the medium.)

At this point in the process, all of the superacid is still buffered bythe sensitizing dye and no unbuffered superacid is available to catalyzethermal decomposition of the secondary acid generator. Accordingly, theimagewise exposure is followed by a second blanket exposure withradiation in the second wavelength range, namely ultra-violet radiationin the process illustrated in Table 2 and FIGS. 2A-2E. As shown in FIG.2C, this second ultra-violet exposure causes further decomposition ofthe superacid precursor and a uniform increase in acid concentrationthroughout the acid-generating layer. The amount of ultra-violetradiation applied in this blanket exposure is chosen so that, in thenon-exposed areas, the acid concentration will not exceed the 2Tthreshold, so that all the acid will remain buffered by the sensitizingdye, but so that, in the exposed areas, the acid concentration willexceed the 2T threshold and unbuffered superacid will be present. FIG.2C and Table 2 show the results obtained using a second ultra-violetexposure sufficient to decompose one mole of superacid precursor.Besides the unchanged secondary acid generator, the exposed areas of theacid-generating layer contain 1 mole of doubly protonated sensitizingdye species, 0.75 mole of unbuffered superacid and 2.25 moles ofsuperacid precursor, while the non-exposed areas contain 0.75 mole ofdoubly protonated sensitizing dye species, 0.25 mole of singlyprotonated sensitizing dye species and 3.25 moles of superacidprecursor.

The remaining two steps of the second preferred process are essentiallyidentical to the corresponding steps of the first preferred process.First, the imaging medium is heated so that in the exposed areas theunbuffered superacid will catalyze the thermal decomposition of thesecondary acid generator, thus converting the 10 moles of secondary acidgenerator to (in theory) 10 moles of secondary acid. In the non-exposedareas, however, since no unbuffered superacid is present, essentially nogeneration of secondary acid occurs. FIG. 2D shows the result of theheating step graphically. Finally, the components of the acid-generatingand color-change layers become intermixed. As in the first preferredprocess described above with reference to Table 1, Table 2 assumes thatthe color-change layer contains 10 moles of an indicator image dye, 1mole of copper compound, 1 mole of a reducing agent (the productsproduced by oxidation of this reducing agent are again ignored in Table2 for simplicity) and 6 moles of a base, shown as potassium acetate inTable 2. Table 2 also again assumes that the image dye is more basicthan the sensitizing dye; the contrary case is discussed below. In thenon-exposed areas, the copper, the reducing agent and the reactivematerial decompose all remaining superacid precursor, with generation ofphenyl acetate (Table 2 assumes), phenyl iodide (omitted from Table 2)and potassium hexafluoroantimonate. In the exposed areas, the potassiumacetate is protonated by the superacid and by some of the secondaryacid. The copper reagent catalyzes decomposition of the remainingsuperacid precursor with the formation of phenyl cations, which reactwith the most nucleophilic species remaining, here assumed to be theimage dye. (In practice, the decompostion of the superacid precursor isprobably somewhat more complicated and other products may be produced;however, the exact type of decomposition products produced does notaffect the essential nature of the present process.)

In the exposed areas, as in the first preferred process, the unbufferedsuperacid and 3.25 moles of the secondary acid are immediatelyneutralized by the potassium acetate, which also deprotonates allprotonated forms of the sensitizing dye to produce the correspondingunprotonated forms. The decomposition of the superacid precursor iscatalyzed by the copper compound, leading to the formation of 2.25 molesof phenylated image dye. 6.75 Moles of secondary acid reversiblyprotonate and form a salt with the image dye. Both the phenylated andthe protonated image dyes are colored. 1 Mole of image dye remainsunprotonated and in its leuco form.

In the non-exposed areas, the potassium acetate deprotonates allprotonated forms of the sensitizing dye to produce the correspondingunprotonated forms, and 1 mole of potassium acetate remains (this freebase is shown as -1 moles of acid if FIG. 2E), so that all the image dyestays in its unprotonated, leuco form and no color change occurs.

One advantage of the present deprotonation process is that, at least inmany preferred embodiments of the invention, it is possible tocompensate for any premature breakdown of the superacid precursor whichmay occur before use of the imaging medium. Such premature breakdown mayoccur, for example, by exposure of the imaging medium to radiationduring transportation and storage or because the mixture of thesuperacid precursor and the sensitizing dye in the acid-generating layerundergoes slow decomposition on protracted storage. If, as in the twopreferred processes described above, the first step of the process isblanket exposure of the imaging medium to radiation in the secondwavelength range to generate superacid and convert the sensitizing dyeto its protonated (first) form, the blanket exposure can be adjusted toensure that the present process works properly, even if somedecomposition of the superacid precursor has occurred earlier.

For example, to take an extreme case purely for purposes ofillustration, suppose that the imaging medium shown in Table 1 isexposed to so much ultra-violet radiation during storage and transportthat premature breakdown of 0.5 mole of superacid precursor occurs. Atthe beginning of imaging, all areas of the medium thus contain 0.5 moleof sensitizing dye, 10 moles of secondary acid generator, 4.5 moles ofsuperacid precursor and 0.5 mole of protonated sensitizing dye. Afterspectral analysis to determine the amount of protonated sensitizing dyealready present, the initial ultra-violet exposure may be adjusted sothat, in exposed areas, only a further 0.25 mole of superacid precursoris decomposed. After this exposure, the medium will contain 0.75 mole ofprotonated sensitizing dye, and will thus be in exactly the samecondition as the medium used in the first preferred process describedabove (at the stage represented in FIG. 1A), in which no prematurebreakdown of the superacid precursor occurred before imaging, but theinitial ultra-violet exposure generated 0.75 mole of superacid. Also,provided that no substantial breakdown of superacid precursor occursduring transportation and storage, a deprotonation medium of the presentinvention which is produced with the sensitizing dye in its unprotonatedform is, prior to imaging, essentially insensitive to radiation of thewavelength used for the imagewise exposure, since the unprotonatedsensitizing dye, even when exposed to such radiation, does not causesubstantial decomposition of the superacid precursor.

For similar reasons, all variants of the present process are relativelyinsensitive to variations in the radiation used for the imagewiseexposure, such as variations in laser output, differences betweenindividual lasers in an array used to form the imaging beam, timingerrors in laser drivers, etc. For example, in the process shown in Table1, the imagewise exposure causes decomposition of 0.75 mole of superacidprecursor. If the imaging radiation delivered to the imaging mediumvaries by ±20%, some exposed areas will experience decomposition of 0.6mole of superacid precursor, while others will experience decompositionof 0.9 mole. Thus, after the imagewise exposure, the concentration ofunbuffered superacid in the exposed areas will vary from 0.35 to 0.65mole. With appropriate control of the heating step, this range ofvariation in unbuffered superacid concentration will have minimaleffects on the final image in cases where the medium is designed to beessentially binary, i.e., any specific pixel is either at D_(min) or atD_(max).

From Tables 1 and 2 and the related description above, it will be seenthat, after the present medium has been imaged and fixed, in both theexposed and non-exposed areas the sensitizing dye has been returned toits unprotonated form. This is always the case in the non-exposed areas,and is also the case in the exposed areas if the image dye issubstantially more basic than the sensitizing dye. If this is not so, inthe exposed areas the sensitizing dye will remain protonated (orpossibly phenylated) and the absorption in the first wavelength range isa combination of that due to the protonated (or phenylated) image dyeand that due to the protonated (or phenylated) sensitizing dye. In suchcases, the sensitizing dye should be chosen so that the presence of itsprotonated (or phenylated) form in the D_(max) areas does not causeobjectionable effects on the image. This is especially important incolor media having a plurality of different acid-generating layers andcolor-change layers since if, for example, the protonated (orphenylated) form of the sensitizing dye used in the acid-generatinglayer associated with the magenta color-change layer has a yellow color,crosstalk will result between the magenta and yellow components of theimage. To reduce or eliminate such objectionable effects, it isdesirable that the protonated (or phenylated) form of the sensitizingdye have a color similar to that of the colored form of the associatedimage dye. Sometimes it may be possible to use the same (or a chemicallysimilar) dye as both the sensitizing dye and the image dye.

The potential problem discussed in the preceding paragraph is confinedto the deprotonation process of the present invention. However, arelated problem occurs in the nucleophile process of the invention ifthe nucleophile is more basic than the image dye.

The sensitizing dye used in the deprotonation process of the presentinvention may be any molecule, the absorption spectrum of which dependsreversibly upon its state of protonation and which can causedecomposition of the superacid precursor used, provided of course thatthe dye is compatible with the other components of the imaging medium.The state of the sensitizing dye called herein the "unprotonated form"need not necessarily be a neutral molecule; the unprotonated form may beanionic but capable of being protonated to a neutral or cationic form.For example, fluorescein monomethyl ether can exist in a non-aqueousmedium in anionic (deprotonated), neutral or cationic (protonated)forms; both the anionic and cationic forms are yellow, while the neutralform is colorless to the eye but absorbs strongly in themid-ultra-violet region (around 320 nm). The spectral shift of thesensitizing dye upon protonation may be either hypsochromic (to shorterwavelength) or bathochromic (to longer wavelength). Fluoresceinmonomethyl ether exhibits both behaviors; the first protonation of theanionic form causes a hypsochromic shift, while the second protonationto the cationic form causes a bathochromic shift.

Preferred indicator sensitizing dyes for use in the deprotonationprocess include fluoran dyes, phthalide dyes, xanthene dyes, acridinedyes, hydroxypyrylium dyes, hydroxythiopyrylium dyes, styrylpyridiniumdyes, styrylquinolinium dyes, and other substituted quinolinium,isoquinolinium and pyridinium dyes, with triarylpyridinium, quinoliniumand xanthene dyes being especially preferred. Specific triarylpyridiniumdyes which have been found useful in the present invention areprotonated forms of:

2,4,6-tris(4-methoxyphenyl)pyridine;

2,6-bis(4-methoxyphenyl)-4-(2-thienyl)pyridine;

2,6-bis(4-methoxyphenyl)-4-(2-(4-bromophenyl)pyridine;

2,6-bis(4-methoxyphenyl)-4-(2-naphthyl)pyridine;

2,4-bis(4-methoxyphenyl)-6-(2-naphthyl)pyridine;

2,4,6-tris(2,4,6-trimethoxyphenyl)pyridine; and

2,6-bis(4-methoxyphenyl)-4-(2-(1,4-dimethoxy)naphthyl)pyridine.

A specific preferred triarylpyridinium dye is the protonated form of2,4,6-tris-(2,4-dimethoxyphenyl)pyridine.

A specific preferred quinolinium dye is the protonated form of 2-[2-[2,4bis[octyloxy]phenyl]ethen-1-yl]quinoline (the unprotonated form of thisdye is available from Yamada Chemical Co., Kyoto, Japan), while aspecific preferred xanthene dye is the protonated form of3',6'-bis[N-[2-chlorophenyl]-N-methylamino]spiro[2-butyl-1,1-dioxo[1,2-benzisothiazole-3(3H),9'-(9H)xanthene]](which may be prepared as described in U.S. Pat. No. 4,345,017).

Methods for the preparation of triarylpyridinium dyes are described inthe literature. One convenient method for the preparation of such dyesbeating identical substituents at the 2- and 6-positions is described inWeiss, J. Am. Chem. Soc., 74, 200 (1952) and comprises heating a mixtureof an acetophenone, an aldehyde (that containing the desired4-substituent) and ammonium acetate in acetic acid. A dihydropyfidine isproduced as the penultimate intermediate, but is oxidized to thepyridine by the intermediate chalcone. A second method is similar to thefirst, but uses hydroxylamine or unsymmetrical dimethylhydrazine inplace of ammonium acetate; the penultimate intermediate in these casesare the N-hydroxydihydropyridine or N,N-dimethylaminodihydropyridine,which undergo elimination and aromatization without the need for anoxidizing agent. A third method, described in Krohnke, Synthesis, 1976,1, can produce asymmetric triarylpyridinium dyes. In this third method,an aryl aldehyde containing the desired 4 substituent and anacetophenone containing the desired 2-substituent are reacted to form anintermediate chalcone, which is then reacted with the phenacylpyridiniumsalt derived from the acetophenone containing the desired 6-substituent.The resultant pyridinium-substituted dihydropyridine undergoes loss ofpyridine with aromatization. All three methods are illustrated inExamples 1-3 below.

The prior art describes various combinations of nucleophiles andsensitizing dyes which can be used in the nucleophile process of thepresent invention; see, for example, U.S. Pat. Nos. 5,258,274 and5,314,795 (although note that in the present process the imaging mediummay contain the nucleophile itself rather than a nucleophile-generatingspecies as in these patents, since the nucleophile can be kept in alayer or phase separate from the acid-generating layer until the finalheating step when the nucleophile converts the sensitizing dye to itssecond form). The nucleophile used in the present process may be aneutral molecule, for example a primary or secondary amine, a stabilizedcarbanion, for example a carbanion derived from a malonate ester or anitroalkane, or a charged nucleophile, for example a thiolate.

The preferred sensitizing dyes for use in the nucleophile process arehemicyanine dyes; the term "hemicyanine dyes" is used herein its broadsense as meaning any dye in which a quaternary nitrogen-containingheterocycle is in charge resonance with an electron-donating moietywithin the same molecule. Hemicyanine dyes which may be used in thepresent process include those described in the aforementioned U.S. Pat.Nos. 5,258,274 and 5,314,795; preferred hemicyanine dyes are those ofthe formula: ##STR1## wherein:

G is a CR^(c) R^(d) group, a CR^(c) =R^(d) group, an oxygen or sulfuratom, or an NR^(b) group;

R^(a) and R^(b) are each an alkyl group containing from about 1 to about20 carbon atoms;

R^(c) and R^(d) are each a hydrogen atom or an alkyl group containingfrom about 1 to about 20 carbon atoms;

n is 1 or 2;

Ar is an aryl or heterocyclyl group; and

X' is an anion.

The anion X of the dye should be chosen with care having regard to thesuperacid which will be generated during the imaging process. Forexample, it is inadvisable to use iodide, or another anion derived froma weak acid, as the anion of the dye, since the presence of such ananion in the acid-generating layer during imaging will cause thesuperacid generated to protonate the anion, thus leading to theformation of HI, or an acid which is similarly weak in a polymericmedium of low dielectric constant (such as those typically used in theimaging media of the present invention), which cannot effectivelyprotonate the secondary acid generator, and thus does not initiate theacid amplification process. Conveniently, the anion X is chosen to bethe same as that of the superacid precursor; thus, for example, when thepreferred superacid precursor diphenyliodonium hexafluoroantimonate isused, the anion X is conveniently hexafluoroantimonate. Specificpreferred hemicyanine sensitizing dyes which have been found useful inthe present nucleophile process include:

1-methyl-2-[2-[2,4-bis[octyloxy]phenyl]ethen-1-yl]quinoliniumhexafluoroantimonate;

1-methyl-2-[4-diphenylaminophenyl]ethen-1-yl]quinoliniumhexafluoroantimonate;

3,3-dimethyl-1-methyl-2-[2-[9-phenylcarbazol-3-yl]ethen-1-yl]-3H-indoliumhexafluoroantimonate; and

3,3-dimethyl-1-methyl-2-[2-[9-ethylcarbazol-3-yl]ethen-1-yl]-3H-indoliumhexafluoroantimonate.

In the present process, it is desirable that the layer or phasecontaining the sensitizing dye also comprise a cosensitizer which is areducing agent less basic than the secondary acid generator, since ithas been found that the presence of such a cosensitizer greatly improvesthe quantum efficiency of the reaction between the photoexcitedsensitizing dye and the superacid precursor (i.e., the quantumefficiency of superacid generation and thus the sensitivity of theimaging medium). Preferred cosensitizers include triarylamines (forexample, triphenylamine) and hydroquinones.

Since the present process relies upon the production of unbufferedsuperacid, it is highly desirable that the process be conducted underessentially anhydrous conditions; as chemists are well aware, the mostpowerful acidic species that can exist in the presence of more than oneequivalent of water is the hydroxonium (hydronium) ion, [H₃ O]⁺.Accordingly, if the medium in which the present process is conductedcontains water, at least part of the superacid produced by the presentprocess will simply generate hydroxonium ion. However, in the absence ofwater, the superacid yields an acidic species much stronger thanhydroxonium ion, and this acidic species can effect the acid-catalyzeddecomposition of various secondary acid generators which hydroxonium ioncannot. Typically, the present process is carded out with the superacidprecursor and the sensitizing dye dispersed in a polymeric binder, andsuch binders can readily be chosen to provide an essentially anhydrousenvironment for the process.

In its most general form, the present process requires only that,following the formation of the imagewise distribution of the secondaryacid, the sensitizing dye be converted to its second form in at leastthe non-exposed areas of the medium. Although, at least in theory, theappropriate reagent for effecting this conversion could be supplied froman external source (for example, by spraying a solution of the reagenton to the exposed medium), for practical reasons it is normallydesirable to have the reagent present in the imaging medium prior toexposure, so that the imaging medium provides a self-contained imagingsystem not requiring the use of liquid reagents. Usually, it isconvenient to have the reagent capable of converting the first form ofthe sensitizing dye to its second form present in the color-change layeror phase of the imaging medium, so that the mixing of the components ofthe acid-generating and color-change layers or phases introduces thereagent into the non-exposed areas of the medium, thus converting thesensitizing dye to its second form. However, there are certainapplications of the invention where it may be desirable to include thereagent in a layer of the imaging medium other than the color-changelayer. In particular, for practical reasons, it is generally convenientto prepare imaging media of the invention by first depositing anacid-generating layer from a non-aqueous medium on to a support, andthen to deposit a color-change layer from an aqueous medium on to theacid-generating layer. If this process is used to prepare a "dupe" filmfor use in the graphic arts industry, it is necessary that the medium beexposed from the color-change layer side, since exposing through therelatively thick support may lead to a loss in resolution. However,exposing through the color-change layer may require that the image dye(which in a write-white medium such as dupe film must be colored in baseand essentially colorless in acid) be present in its colorless formduring the exposure, since otherwise it may absorb the radiationintended for exposure of the underlying acid-generating layer. To ensurethat the image dye is coated in a form which does not prevent imaging byabsorbing the imaging radiation, a medium of this type convenientlycomprises:

(a) a color-change layer comprising an image dye which has substantialabsorption (i.e., is "colored," where the "color" in question willtypically be in the near ultra-violet) in base but has low absorption inacid, the image dye being present in its protonated (colorless) form;

(b) an acid-generating layer; and

(c) a layer containing a conversion reagent (typically a base)interposed between the acid-generating layer and the color-change layer,the quantity of the conversion reagent being sufficient to causecoloration of all the image dye in non-exposed regions of the mediumafter heating, plus any additional amount necessary for fixation.

In principle, in the present process the mixing of the components of theacid-generating and color-change layers should be effected after thegeneration of the secondary acid from the secondary acid generator.However, in practice both the generation of the secondary acid in theacid-generating layer and the mixing of the components of the two layersmay be effected in a single heating step, since the superacid-catalyzeddecomposition of the secondary acid generator will typically beessentially complete before mixing of the two layers becomessignificant.

Obviously, it is important that the components of the acid-generatinglayer and the color-change layer not mix prematurely. In practice, asalready noted, the present imaging medium will typically be formed bycoating both layers from a solution or dispersion on to a support. Toprevent premature mixing, it is usually desirable to coat one layer froman aqueous medium and the other from a non-aqueous medium. Typically,the acid-generating layer is coated from an organic medium and thecolor-change layer from an aqueous medium.

Any of the known superacid precursors, for example diazonium,phosphonium, sulfonium and iodonium compounds, may be used in thisinvention, but iodonium compounds are preferred. Especially preferredsuperacid precursors are diphenyliodonium salts, specifically(4-octyloxyphenyl)phenyliodonium hexafluorophosphate andhexafluoroantimonate, bis(n-dodecylphenyl)iodonium hexafluoroantimonateand (4-(2-hydroxytetradecan-1-yloxy)phenyl)phenyl iodoniumhexafluoroantimonate.

Any secondary acid generator capable of superacid-catalyzed breakdown togive a secondary acid may be used in the present process. One preferredgroup of secondary acid generators is3,4-disubstituted-cyclobut-3-ene-1,2-diones (hereinafter for conveniencecalled "squaric acid derivatives") capable of generating squaric acid oran acidic derivative thereof, since squaric acid and its acidicderivatives are strong acids well suited to effecting color changes inacid-sensitive materials. Especially preferred squaric acid derivativesare those in which there is bonded to the squaric acid ring, via anoxygen atom, an alkyl or alkylene group, a partially hydrogenated arylor arylene group, or an aralkyl group. The acid-catalyzed decompositionof these squaric acid derivatives causes replacement of the originalalkoxy, alkyleneoxy, aryloxy, aryleneoxy or aralkoxy group of thederivative with a hydroxyl group, thus producing squaric acid or anacidic squaric acid derivative having one hydroxyl group.

The exact mechanism by which squaric acid or an acidic derivativethereof is formed in the present process may vary depending upon thetype of squaric acid derivative employed. In some cases, for exampledi-t-butyl squarate, one or both groups attached via oxygen atoms to thesquaric acid ring may thermally decompose to yield an alkene or arene,thus converting an alkoxy or aryloxy group to a hydroxyl group andforming the squaric acid or acidic derivative thereof. In other cases,for example 3-amino-4-(p vinylbenzyloxy)cyclobut-3-ene-1,2-dione, thereis no obvious mechanism for formation of a corresponding alkene orarene, and it appears that the mechanism of acid formation is migrationof the vinylbenzyl carbocation or similar group to a different positionwithin the molecule (probably to the amino group), and protonation ofthe remaining oxygen atom to form a hydroxyl group at the position fromwhich the group migrates. In other cases, neither of these pathways ispossible. However, in all cases the net effect is the replacement of thealkoxy, alkyleneoxy, aryloxy, aryleneoxy or aralkoxy group present inthe original derivative with a hydroxyl group to form squaric acid or anacidic derivative thereof.

Those skilled in the art of organic chemistry will appreciate that thesusceptibility to thermal decomposition of the squaric acid derivativespreferred for use in the present process is related to the stability ofthe cation produced from the ester grouping during the decompositionprocess. Although the stability of specific cations may be influenced bya variety of factors, including steric factors, which may be peculiar toa particular ester, in general it may be said that the squaric acidesters preferred for use in the present process are:

(a) primary and secondary esters of squaric acid in which the α-carbonatom (i.e., the carbon atom bonded directly to the -O- atom of thesquarate ring) bears a non-basic cation-stabilizing group. Thiscation-stabilizing group may be, for example, an sp² or sp hybridizedcarbon atom, or an oxygen atom;

(b) tertiary esters of squaric acid in which the α-carbon atom does nothave an sp² or sp hybridized carbon atom directly bonded thereto; and

(c) tertiary esters of squaric acid in which the α-carbon atom does havean sp² or sp hybridized carbon atom directly bonded thereto, providedthat this sp² or sp hybridized carbon atom (or at least one of these sp²or sp hybridized carbon atoms, if more than one such atom is bondeddirectly to the α-carbon atom) is conjugated with anelectron-withdrawing group.

It will be apparent to skilled organic chemists that, provided one ofthe aforementioned types of ester groupings is present in the squaricacid derivative to produce one hydroxyl group after thermaldecomposition, the group present in place of the other hydroxyl group ofsquaric acid is of little consequence, provided that this other groupdoes not interfere with the thermal decomposition. Indeed, the widevariation possible in this other group has the advantage that this groupcan be varied to control other properties of the derivative, for exampleits compatibility with other components of the imaging medium, or itssolubility in solvents used to form coating solutions used in thepreparation of the imaging medium.

Examples of squaric acid derivatives useful in the present processesinclude:

(a) those of the formula: ##STR2## in which R¹ is an alkyl group, apartially hydrogenated aromatic group, or an aralkyl group, and R² is ahydrogen atom or an alkyl, cycloalkyl, aralkyl, aryl, amino, acylamino,alkylamino, dialkylamino, alkylthio, alkylseleno, dialkylphosphino,dialkylphosphoxy or trialkylsilyl group, subject to the proviso thateither or both of the groups R¹ and R² may be attached to a polymer.Among the derivatives of Formula I, especially preferred groups arethose in which (a) R¹ is an unsubstituted or phenyl substituted alkylgroup containing a total of not more than about 20 carbon atoms, and R²is an alkyl group containing not more than about 20 carbon atoms, or aphenyl group (which may be substituted or unsubstituted); and (b) R¹ isa benzyl group and R² is an amino group.

(b) those of the formula: ##STR3## in which R¹ and R³ independently areeach an alkyl group, a partially hydrogenated aryl group or an aralkylgroup, subject to the proviso that either or both of the groups R¹ andR³ may be attached to a polymer. Among the derivatives of Formula II, anespecially preferred group are those in which R¹ and R³ are eachindependently an unsubstituted or phenyl substituted alkyl groupcontaining a total of not more than about 20 carbon atoms. Specificpreferred compounds of Formula II are those in which R¹ and R³ are eacha tertiary butyl group, a benzyl group, an α-methylbenzyl group or acyclohexyl group, namely di-tertiary butyl squarate, dibenzyl squarate,bis(α-methylbenzyl) squarate and dicyclohexyl squarate.

(c) those of the formula: ##STR4## in which n is 0 or 1, and R⁴ is analkylene group or a partially hydrogenated arylene group. Among thederivatives of Formula III, an especially preferred group are those inwhich n is 1 and R⁴ is an alkylene group containing not more than about12 carbon atoms.

(d) those having at least one unit of the formula: ##STR5## in which nis 0 or 1, and R⁵ is an alkylene or partially hydrogenated arylenegroup. Besides the fragmentable groups R⁵, the compounds may alsocontain one or more units in which a non-fragmentable group is attachedto a squarate ring, directly or via an oxygen atom.

The squaric acid derivatives of Formula IV include not only highpolymers, but also dimers, trimers, tetramers, etc. including at leastone of the specified units. The terminating groups on the derivatives ofFormula IV may be any of groups OR¹ or R² discussed above with referenceto Formula I. Thus, for example, Formula IV includes the squaric aciddimer derivative of the formula: ##STR6##

The squaric acid derivatives of Formulae I and II are usually monomeric.However, these derivatives of Formulae I and II can be incorporated intopolymers by having at least one of the groups R¹, R² and R³ attached toa polymer. Attachment of the squaric acid derivatives to a polymer inthis manner may be advantageous in that it may avoid incompatibilityand/or phase separation that might occur between a monomeric squaricacid derivative of Formula I or II and a polymeric binder needed in animaging medium.

The attachment of the groups R¹, R² and R³ to a polymer may be effectedin various ways, which will be familiar to those skilled in the art ofpolymer synthesis. The squaric acid derivatives may be incorporated intothe backbone of a polymer, for example in a polymer similar to the dimerof the formula given above. Alternatively, the squaric acid derivativesmay be present as sidechains on a polymer; for example, one of thegroups R¹, R² and R³ could contain an amino group able to react with apolymer containing carboxyl groups or derivatives thereof to form anamide linkage which would link the square acid derivative as a sidechainon to the polymer, or these groups may contain unsaturated linkages thatenable the squaric acid derivatives to be polymerized, either alone orin admixture with other unsaturated monomers.

In the present process, it is generally undesirable to form substantialquantities of gas during the acid-catalyzed decomposition of thesecondary acid generator since such gas may distort the imaging mediumor form vesicles therein, and such distortion or vesicle formation mayinterfere with proper image formation. Accordingly, if the decompositionof the squaric acid derivative yields an alkene, it is desirable thatthe groups R¹, R³, R⁴ and R⁵ be chosen so that this alkene is a liquidat 20° C., and preferably higher, since some heating of the alkene willinevitably occur during the acid-catalyzed decomposition. Sometimes,however, the alkene liberated may be sufficiently soluble in the mediumcontaining the squaric acid derivative that liberation of a highlyvolatile alkene will not result in distortion of, or vesicle formationin, the medium.

Another preferred group of secondary acid generators for use in thepresent process are oxalic acid derivatives that undergo acid-catalyzedbreakdown to give oxalic acid or an acidic derivative thereof, forexample an oxalic acid hemiester. Although oxalic acid and its acidicderivatives are not quite such strong acids as squaric acid and itsacidic derivatives, oxalic acid and its derivatives are sufficientlystrong acids for use with most image dyes. Also, oxalic acid derivativesare, in general, less costly than squaric acid derivatives.

The types of oxalic acid derivatives preferred for use in the presentprocess are rather more diverse in structure than the squaric acidderivatives, and the choice of oxalic acid derivative for any specificprocess may be governed more by the thermal breakdown properties of thederivative than its exact chemical structure; in general, for practicalreasons such as the limited temperature range to which other componentsof the imaging medium may safely be exposed, it is preferred that theoxalic acid derivative be one which begins to decompose thermally at atemperature in the range of about 140° to about 180° C., as measured bydifferential scanning calorimetry in a nitrogen atmosphere at a 10°C./minute temperature ramp, in the absence of any catalyst. Since thepresence of an acid catalyst lowers the thermal decompositiontemperature of oxalic acid derivatives by at least about 20° C. andpotentially significantly more, derivatives which decompose uncatalyzedat about 140° to about 180° C., will, in the presence of acid, decomposeat temperatures as low as about 65° C., temperatures to which othercomponents of the imaging medium can in general be exposed.

The factors affecting the ability of the oxalic acid derivatives toundergo acid-catalyzed thermal decomposition are similar to thoseaffecting the ability of the aforementioned squaric acid derivatives toundergo the same reaction, and thus the preferred ester groups are ofthe same types. Accordingly, preferred oxalic acid derivatives for usein the present process include:

(a) primary and secondary esters of oxalic acid in which the α-carbonatom (i.e., the carbon atom bonded directly to the -O- atom of theoxalate grouping) bears a non-basic cation-stabilizing group. Thiscation-stabilizing group may be, for example, an sp² or sp hybridizedcarbon atom, or an oxygen atom;

(b) tertiary esters of oxalic acid in which the α-carbon atom does nothave an sp² or sp hybridized carbon atom directly bonded thereto; and

(c) tertiary esters of oxalic acid in which the α-carbon atom does havean sp² or sp hybridized carbon atom directly bonded thereto, providedthat this sp² or sp hybridized carbon atom (or at least one of these sp²or sp hybridized carbon atoms, if more than one such atom is bondeddirectly to the α-carbon atom) is conjugated with anelectron-withdrawing group.

(d) an ester formed by condensation of two moles of an alcohol with thebis(hemioxalate) of a diol, provided that the ester contains at leastone ester grouping of types (a), (b) or (c) above. One example of anester of this type is that of the structure: ##STR7## which can beregarded as formed from two moles of menthol(2-methylethyl-4-methylcyclohexanol) and one mole of thebis(hemioxalate) of 1,6-bis-(4-hydroxymethylphenoxy)hexane. Since thestructure of the central residue of the diol in such esters can varywidely, the solubility and other properties of the esters can be "tuned"as required for compatibility with other components of the imagingmedium, while the nature of the end groups, which undergo theacid-forming thermal decomposition, can be varied independently of thenature of the central residue.

(e) polymeric oxalates derived from polymerization of oxalate estershaving an ethylenically unsaturated group, provided that the estercontains at least one ester grouping of type (a), (b) or (c) above. Aswith the squaric acid derivatives discussed above, use of a polymericoxalate rather than a monomeric one may be advantageous in that it mayavoid incompatibility and/or phase separation that might occur between amonomeric derivative and a polymeric binder needed in an imaging medium.Use of a polymeric derivative also tends to inhibit diffusion of theoxalate through the imaging medium during storage before imaging.Although polymeric oxalates can be formed in other ways, at present weprefer to form such oxalates by first forming an oxalate ester in whichone of the ester groupings comprises an ethylenically unsaturated group,and then polymerizing this ester using a conventional free radicalpolymerization initiator, for example azobis(isobutyronitrile) (AIBN).The ethylenically unsaturated group is conveniently an acrylate ormethacrylate group, while the other ester grouping in the monomericoxalate can be any of the types discussed above.

(f) Condensation polymers of oxalates, provided that the ester containsat least one ester grouping of type (a), (b) or (c) above. This type ofpolymer also possesses the advantages discussed under (e) above.

Methods for the synthesis of the preferred secondary acid generatorsdescribed above are given in the aforementioned U.S. Pat. No. 5,286,612,application Ser. No. 08/141,852, and International Application No.PCT/US93/10215.

The image dye used in the present invention may be any material thatundergoes a color change in the presence of the secondary acid. Thus anyconventional indicator dye may be used as the acid-sensitive material,as may the leuco dyes disclosed in U.S. Pat. Nos. 4,602,263; 4,720,449and 4,826,976, which are also sensitive to acid.

As will be apparent to those skilled in the imaging art, a medium of thepresent invention which contains a plurality of color-change layers (forexample, a full color medium containing three or four color-changelayers) need not use the present process in all of the color-changelayers; one or more of the color-change layers can use the presentprocess, while the other color-change layer(s) use other color-formingmechanisms, for example the '612 process or conventional sensitizationof superacid precursors with non-basic polycyclic aromatic sensitizers.For example, the specific preferred embodiment of the inventiondescribed below with reference to FIG. 3 uses two acid-generating andtwo color-change layers using the present process, and oneacid-generating and color-change layer using a conventional non-basicpolycyclic aromatic sensitizer.

It should be noted that the present process may allow the use ofcombinations of superacid precursor and indicator sensitizing dye inwhich the combination of the precursor and the absorbing (i.e., first)form of the sensitizing dye is unstable on long term storage. Providedthat the combination of the precursor and the non-absorbing (i.e.,second) form of the sensitizing dye is stable on long term storage, therelevant acid-generating layer can be prepared with the dye in itssecond (or other precursor) form, and the first form of the dye producedimmediately before the imagewise exposure; in a deprotonation process,the conversion of the second form of the sensitizing dye to its firstform is conveniently accomplished by exposing the imaging medium toactinic radiation effective to generate superacid within theacid-generating layer, as described above with reference to Table 1.After the imagewise exposure, the conversion of the sensitizing dye toits second form in accordance with the present invention and/or fixingeffected by destruction of the remaining superacid precursor will ensurethat the unstable combination of the superacid precursor and the firstform of the sensitizing dye is not present after imaging.

Preferred uses of the present process include:

(a) the use of visible imagewise exposure to produce a visible image,which may be positive or negative;

(b) a true- or false-sensitized full color image exposed at threedifferent wavelengths (for example, a print paper)

(c) the use of near infra-red (700-1200 nm) radiation to produce avisible image having good D_(min) when viewed in reflection (in thisprocess, the first from of the sensitizing dye has a near infra-redabsorption peak and the second form of the dye has a substantially lowervisible absorption than the first form);

(d) the use of ultra-violet exposure to form an ultra-violet photomask;and

(e) the formation of a full color image using a single source(preferably a laser) at a single visible or near infra-red wavelength toeffect imagewise exposure of all three acid-generating layers of themedium.

Process (e) above conveniently uses a deprotonation process of theinvention with an imaging medium having three associated pairs ofacid-generating layers and color-change layers (each pair comprising anacid-generating layer and a color-change layer may hereinafter be calleda "bilayer"), with each adjacent pair of bilayers being separated by anacid-impermeable interlayer. This type of imaging medium comprises:

a first acid-generating layer comprising a sensitizing dye in itsprotonated form, optionally a cosensitizer, a superacid precursor and asecondary acid generator;

a first color-change layer disposed adjacent the first acid-generatinglayer and comprising a reactive material, a copper compound and a firstimage dye undergoing a change in its absorption of radiation uponcontact with the secondary acid generated upon acid-catalyzeddecomposition of the secondary acid generator in the firstacid-generating layer;

a first acid-resistant interlayer superposed on the firstacid-generating layer and the first color-change layer;

a second acid-generating layer disposed on the opposed side of the firstacid-resistant interlayer from the first acid-generating layer and thefirst color-change layer, the second acid-generating layer comprising asensitizing dye in its unprotonated form, optionally a cosensitizer, asuperacid precursor and a secondary acid generator, the secondacid-generating layer further comprising a first auxiliary sensitizerwhich renders the superacid precursor therein susceptible todecomposition by actinic radiation of a first wavelength in the secondwavelength range, but not susceptible to decomposition by actinicradiation of a second wavelength in the second wavelength range;

a second color-change layer disposed adjacent the second acid-generatinglayer and on the opposed side of the first acid-resistant interlayerfrom the first acid-generating layer and the first color-change layer,the second color-change layer comprising a reactive material, a coppercompound and a second image dye undergoing a change in its absorption ofradiation upon contact with the secondary acid generated uponacid-catalyzed decomposition of the secondary acid generator in thesecond acid-generating layer, the absorption change undergone by thesecond image dye being different from that undergone by the first imagedye;

a second acid-resistant interlayer disposed on the opposed side of thesecond acid-generating layer and second color-change layer from thefirst acid-resistant interlayer;

a third acid-generating layer disposed on the opposed side of the secondacid-resistant interlayer from the second acid-generating layer andsecond color-change layer, the third acid-generating layer comprising asensitizing dye in its unprotonated form, optionally a cosensitizer, asuperacid precursor and a secondary acid generator, the thirdacid-generating layer further comprising a second auxiliary sensitizerwhich renders the superacid precursor therein susceptible todecomposition by actinic radiation of the second wavelength in thesecond wavelength range, but not susceptible to decomposition by actinicradiation of the first wavelength in the second wavelength range; and

a third color-change layer disposed adjacent the third acid-generatinglayer and on the opposed side of the second acid-resistant interlayerfrom the second acid-generating layer and the second color-change layer,the third color-change layer comprising a reactive material, a coppercompound and a third image dye undergoing a change in its absorption ofradiation upon contact with the secondary acid generated uponacid-catalyzed decomposition of the secondary acid generator in thethird acid-generating layer, the absorption change undergone by thethird image dye being different from those undergone by the first andsecond image dyes. Very conveniently, in this preferred form of imagingmedium, the same sensitizing dye, superacid precursor and secondary acidgenerator are present in each of the three acid-generating layers. Ifthe image is to be fixing, the same fixed reagent is also preferablyused in each of the three color-forming layers.

This type of imaging medium is imaged in the following manner. First,the medium is imagewise exposed, from the surface closer to the thirdacid-generating layer, to actinic radiation in the first wavelengthrange, thereby causing, in the exposed areas of the firstacid-generating layer or phase, the sensitizing dye to decompose atleast part of the superacid precursor, with formation of unbufferedsuperacid in the first acid-generating layer, without substantialproduction of unbuffered superacid in the second and thirdacid-generating layers. Thereafter, the whole imaging medium is exposedto radiation of the first wavelength in the second wavelength range,thus decomposing part of the superacid precursor in the secondacid-generating layer to produce superacid and convening at least partof the sensitizing dye in the second acid-generating layer to itsprotonated form, without substantial production of superacid in thethird acid-generating layer. The medium is then imagewise exposed toactinic radiation in the first wavelength range, thus causing, in theexposed areas of the second acid-generating layer or phase, thesensitizing dye to decompose at least part of the superacid precursor,with formation of unbuffered superacid in the second acid-generatinglayer, without substantial production of unbuffered superacid in thefirst and third acid-generating layers. Thereafter, the whole imagingmedium is exposed to radiation of the second wavelength in the secondwavelength range, thus decomposing part of the superacid precursor inthe third acid-generating layer to produce superacid and converting atleast part of the sensitizing dye in the third acid-generating layer toits protonated form. The medium is then imagewise exposed to actinicradiation in the first wavelength range, thus causing, in the exposedareas of the third acid-generating layer or phase, the sensitizing dyeto decompose at least part of the superacid precursor, with formation ofunbuffered superacid in the third acid-generating layer, withoutsubstantial production of unbuffered superacid in the first and secondacid-generating layers. The last two stages of the imaging process areheating the medium to cause, in the exposed areas of the first, secondand third acid-generating layers, acid-catalyzed thermal decompositionof the secondary acid generator and formation of the secondary acid, andadmixing the components of the first acid-generating layer with those ofthe first color-change layer, the components of the secondacid-generating layer with those of the second color-change layer, andthe components of the third acid-generating layer with those of thethird color-change layer, thus causing, in the areas of the mediumexposed to the three imagewise exposures, the secondary acids to bringabout the changes in absorption of the first, second and third imagedyes and thus form a trichrome image, and the copper compound andreactive material to destroy the remaining superacid precursor in eachof the three acid-generating layers, thus fixing the image. If the samesensitizing dye is used in each of the three acid-generating layers, allthree imagewise exposures can be effected using radiation of the samewavelength (for example, a single laser) thus avoiding, for example, theneed for three separate sources of imaging radiation all of which mustbe scanned across the imaging medium.

Besides the acid-generating and color-change layers or phases, theimaging media of the present invention may comprise a support andadditional layers, for example, a subbing layer to improve adhesion tothe support, acid-impermeable interlayers (as discussed above) forseparating multiple bilayers from one another, an anti-abrasive topcoatlayer, and other auxiliary layers.

The support employed may be transparent or opaque and may be anymaterial that retains its dimensional stability at the temperature usedfor image formation. Suitable supports include paper, paper coated witha resin or pigment, such as, calcium carbonate or calcined clay,synthetic papers or plastic films, such as polyethylene, polypropylene,polycarbonate, cellulose acetate and polystyrene. The preferred materialfor the support is a polyester, desirably poly(ethylene terephthalate).

Usually the acid-generating and color-change layers or phases will eachalso contain a binder; typically these layers are formed by combiningthe active materials and the binder in a common solvent, applying alayer of the coating composition to the support and then drying. Ratherthan a solution coating, the layer may be applied as a dispersion or anemulsion. The coating composition also may contain dispersing agents,plasticizers, defoaming agents, coating aids and materials such as waxesto prevent sticking.

The binder used for the acid-generating layer(s) must of course benon-basic, such that the superacid is not buffered by the binder.Examples of binders that may be used include styrene-acrylonitrilecopolymers, polystyrene, poly(α-methylstyrene), copolymers of styreneand butadiene, poly(methyl methacrylate), copolymers of methyl and ethylacrylate, poly(vinyl butyral), polycarbonate, poly(vinylidene chloride)and poly(vinyl chloride). It will be appreciated that the binderselected should not have any adverse effect on the superacid precursor,sensitizing dye, secondary acid generator, fixing reagent (if any) orimage dye incorporated therein. Also, the binder should be heat-stableat the temperatures encountered during the heating step and should betransparent so that it does not interfere with viewing of the image. Thebinder must of course transmit the actinic radiation used in theexposure steps.

The imaging media of the present invention may be used in any of theways in which the aforementioned '489 and '612 media have been used.Specifically, the imaging media of the present invention are verysuitable for use in slide blanks similar to those described in copendingapplications Ser. Nos. 08/226,452 and 08/226,657, both filed Apr. 12,1994 and assigned to the same assignee as the present application (nowU.S. Pat. Nos. 5,422,230 and 5,451,478 respectively) and thecorresponding International Applications Nos. PCT/US95/04401 andPCT/US95/04395 respectively (Publication Nos. WO 95/27623 and WO95/27622 respectively).

Thus, one preferred slide blank of the present invention comprises:

a support;

a mask layer having a substantially transparent central portion and anon-transparent peripheral portion surrounding the central portion; and,

an imageable layer comprising an imaging medium of the present inventionwhich is imageable to form an image which can be viewed in transmission,

the support, mask layer and imageable layer being secured together sothat the support and the imageable layer extend across essentially theentire transparent central portion of the mask layer, at least theportion of the support adjacent the central portion of the mask layerbeing substantially transparent.

A second preferred slide blank of the present invention comprises:

a support at least part of which is essentially transparent;

an imageable layer superposed on one face of the support, the imageablelayer comprising an imaging medium of the present invention which isimageable to form an image which can be viewed in transmission; and

a protective layer superposed on the imageable layer on the opposed sidethereof from the support, at least part of the protective layer beingessentially transparent;

the support, imageable layer and protective layer being secured togetherto form a slide blank having a thickness of at least about 0.8 mm, andthe thickness of the protective layer being such that no part of theimageable layer containing the color-forming composition is more thanabout 0.2 mm from one external surface of the slide blank.

For further details of these preferred slide blanks, methods for theiruse and slides produced therefrom, the readier is referred to theaforementioned applications Ser. Nos. 08/226,452 and 08/226,657, andInternational Applications Nos. PCT/US95/04401 and PCT/US95/04395.

A preferred embodiment of the invention will now be described, though byway of illustration only, with reference to FIG. 3 of the accompanyingdrawings, which shows a schematic cross-section through a full colordeprotonation type imaging medium (generally designated 10) of theinvention as the image therein is being fixed by being passed between apair of hot rollers 12.

The imaging medium 10 comprises a support 14 formed from a plastic film.Typically the support 14 will comprise a polyethylene terephthalate film3 to 10 mils (76 to 254 mμ) in thickness, and its upper surface (in FIG.3) may be treated with a sub-coat, such as are well-known to thoseskilled in the preparation of imaging media, to improve adhesion of theother layers to the support.

On the support 14 is disposed a first acid-generating layer 16comprising:

(a) a superacid precursor, namely (4-octyloxyphenyl)phenyliodoniumhexafluoroantimonate;

(b) an indicator sensitizing dye of the formula: ##STR8## (theunprotonated form is available from Yamada Chemicals, Kyoto, Japan);this sensitizing dye sensitizes the superacid precursor to visibleradiation at approximately 450 nm);

(c) a secondary acid generator, which undergoes a superacid-catalyzedthermal decomposition to form a secondary acid, this secondary acidgenerator being of the formula: ##STR9##

(d) a cosensitizer, preferably triphenylamine; and

(e) a polystyrene binder.

On the opposed side of the acid-generating layer 16 from the support 14is disposed a first color-change layer 18 comprising:

(a) a first image dye, of the formula: ##STR10## (available from HiltonDavis Co., 2235 Langdon Farm Road, Cincinnati, Ohio 45237 under thetradename "Copikem 37"), which changes from colorless to yellow in thepresence of an acid;

(b) copper(II) acetate;

(c) a reactive material, namely potassium acetate; and

(d) a binder comprising Acryloid B82 (available from Rohm & Haas,Philadelphia, Pa. 19104) and poly(vinyl alcohol); the poly(vinylalcohol) acts as both a binder and a reducing agent for the fixingprocess.

The acid-generating layer 16 and the color-change layer 18 both containa binder having a glass transition temperature substantially above roomtemperature.

Superposed on the first color-change layer 18 is an acid-impermeablelayer 20, which serves to prevent acid generated in the secondacid-generating layer 22 (see below) during imaging penetrating to thefirst color-change layer 18. Superposed on the acid-impermeable layer 20is a second acid-generating layer 22, which contains the same superacidprecursor, secondary acid generator and binder as the firstacid-generating layer 16. However, the second acid-generating layer 22contains, in its protonated form, as an indicator sensitizing dye,2,4,6-tris(2,4-dimethoxyphenyl)pyridine, which sensitizes the superacidprecursor to visible/near ultra-violet radiation of approximately 400 nmwavelength.

Superposed on the second acid-generating layer 22 is a secondcolor-change layer 24 which is identical to the first color-changelayer, except that the Copikem 37 is replaced by a second image dye, ofthe formula: ##STR11## (available from Hilton Davis Co. under thetradename "Copikem 35"), which changes from colorless to magenta in thepresence of an acid.

The next layer of the imaging medium is a second acid-impermeableinterlayer 26, identical to the layer 20. Superposed on theacid-impermeable layer 26 is a third acid-generating layer 28, whichcontains the same superacid precursor, secondary acid generator andbinder as the first and second acid-generating layers 16 and 22respectively. However, this third acid-generating layer 28 does notcontain an indicator sensitizing dye, but instead contains aconventional non-basic polycyclic aromatic sensitizer, namely1-vinylpyrene, which sensitizes the superacid precursor to ultra-violetradiation of approximately 350 nm wavelength. Superposed on the thirdacid-generating layer 28 is a third color-change layer 30 which isidentical to the first color-change layer, except that the Copikem 37 isreplaced by a third image dye, of the formula: ##STR12## (see U.S. Pat.No. 4,345,017) which changes from colorless to cyan in the presence ofan acid. Finally, the imaging medium 10 comprises an abrasion-resistanttopcoat 32.

The imaging medium 10 is exposed by writing on selected areas of themedium with three radiation sources having wavelengths of 450, 400 and350 nm respectively. The three radiation sources may be appliedsimultaneously or sequentially; for example, the medium may be scannedin a raster pattern simultaneously by the focused beams from threelasers of appropriate wavelengths, or the medium may be exposedsequentially through three masks to radiation from lamps generatingradiation of appropriate wavelengths. The 450 nm radiation, whichcarries the yellow channel of the desired image, images the firstacid-generating layer 16, the 400 nm radiation, which carries themagenta channel, images the second acid-generating layer 22 and the 350nm radiation, which carries the cyan channel, images the thirdacid-generating layer 28. Thus, as described above with reference toFIGS. 1A-1D, since the sensitizing dyes in the first and secondacid-generating layers 16 and 22 respectively are present in protonatedform (i.e., these two layers are, prior to imaging, as shown in FIG. 1A,except that all the sensitizing dye, not merely 75%, is present inprotonated form), latent images in unbuffered superacid are formed inthe first and second acid-generating layers 16 and 22. A latent image inunbuffered superacid is also present in the third acid-generating layer28, since the vinylpyrene sensitizer used in this layer does not bufferthe superacid produced by decomposition of the superacid precursor.

The imaging medium 10 is passed between the heated rollers 12; the heatapplied by these rollers causes the unbuffered superacid present in theexposed areas of the acid-generating layers 16, 22 and 28 to causecatalytic breakdown of the secondary acid generator therein, thuscausing formation of a quantity of secondary acid substantially greaterthan the quantity of unbuffered superacid generated by the imagewiseexposures. The heat and pressure applied by the heated rollers 12 alsoraise the acid-generating layers 16, 22 and 28 and the color-changelayers 18, 24 and 30 above their glass transition temperatures, thuscausing the components present in each acid-generating layer to intermixwith the components present in its associated color-change layer.Accordingly, the three associated pairs of acid-generating andcolor-change layers are "developed" and fixed as described above withreference to Table 1; i.e., the copper compound decomposes the remainingsuperacid precursor and the base neutralizes the unbuffered superacidpresent. In these exposed areas, the secondary acid produced in theacid-generating layer effects the color change of the image dye in theassociated color-change layer, thereby forming yellow, magenta and cyanimages in the layers 18, 24 and 30. In the non-exposed areas, excessbase remains and the image dye remains uncolored. The acid-impermeableinterlayers 20 and 26 prevent the unbuffered superacid or the secondaryacid generated in the second and third acid-generating layers 22 and 28respectively migrating to the first and second color-change layers 18and 24 respectively, thus preventing crosstalk among the three images.The mixing of the components present in each bilayer also causes thebase present in each of the color-change layers to deprotonate theprotonated forms of the sensitizing dye (in the layers using indicatorsensitizing dye) present in the non-exposed areas of its associatedacid-generating layer, thus removing the visible absorption due to theprotonated sensitizing dye, and reducing the D_(min) of the images to alow level.

The following Examples are now given, though by way of illustrationonly, to show details of preferred reagents, conditions and techniquesfor use in the process and medium of the present invention.

EXAMPLES 1-7: Deprotonation Processes EXAMPLES 1-3: Preparation ofsensitizing dyes for deprotonation processes EXAMPLE 1: Preparation of2,4,6-tris(4-methoxyphenyl)pyridine

This Example illustrates the first of the three preferred methods forthe preparation of triarylpyridinium dyes described above.

A mixture of p-anisaldehyde (4.08 g, 30 mmole), p-methoxyacetophenone(9.01 g, 60 mmole) and ammonium acetate (30 g) in acetic acid (75 mL)was stirred at reflux for one hour, then at 20° C. for 16 hours. Theprecipitate formed was collected by filtration, washed with a 70:30acetic acid/water mixture, and dried in vacuo to provide 3.50 g (30%yield based upon starting p-methoxyacetophenone) of crude product.Recrystallization from ethanol provided fine colorless matted needles,m. pt. 132°-133.5° C. (Dilthey, J. Pract. Chim. [2], 102, 239 gives themelting point as 133° C.).

EXAMPLE 2: Preparation of 2,4,6-tris(2,4-dimethoxyphenyl)pyridine

This Example illustrates the second of the three preferred methods forthe preparation of triarylpyridinium dyes described above.

A mixture of 2,4-dimethoxybenzaldehyde (1.66 g, 10 mmole),2,4-dimethoxyacetophenone (3.60 g, 20 mmole) and potassium acetate (10.0g) in acetic acid (35 mL) was stirred with heating until reflux wasreached, then allowed to cool to 20° C. To the resultant yellow-orangesolution was added hydroxylamine hydrochloride (1.4 g, 20 mmole). Thesolution was then stirred at reflux for 1 hour, an additional 5.6 g (80mmole) of hydroxylamine hydrochloride was added, and the solution wasagain refluxed for 1 hour. The dark reaction mixture containingsuspended solid thus produced was quenched into water (200 mL) and theresultant solution neutralized with potassium hydroxide. The slurrywhich resulted was extracted with methylene chloride (2×50 mL aliquots)and the organic layer evaporated to a brown oil, which was trituratedwith methanol (80 mL) to provide a cream-colored solid weighing 1.58grams. A second crop weighing 0.41 g was obtained by evaporation of theliquors. Total yield of crude product was 1.99 g (42%).Recrystallization from ethyl acetate provided an analytically puresample, m. pt. 160.5°-162° C.

EXAMPLE 3: Preparation of2,6-bis(2,4-dimethoxyphenyl)-4-(2-(1,4-dimethoxy)naphthyl)pyridine

This Example illustrates the third of the three preferred methods forthe preparation of triarylpyridinium dyes described above.

A mixture of 1,4-dimethoxy-2-naphthaldehyde (1.08 g, 5 mmole, preparedas described in N. P. Buu-Hoi and D. Lavit, J. Chem. Soc. 1956, 1743),p-methoxyacetophenone (0.75 g, 5 mmole) and methanesulfonic acid (12drops) in acetic acid (10 mL) was stirred at reflux for 70 minutes, thencooled to 20° C. Ammonium acetate (4.0 g) and4-methoxyphenacylpyridinium bromide (1.54 g, 5 mmole) were added and thereaction mixture was refluxed for a further 2 hours. The mixture wasthen cooled to 20° C. and poured into water. The resultant solution wasneutralized with sodium hydroxide, and extracted with methylene chloride(20 mL). The organic layer was washed with water (90 mL) and evaporatedto a brown solid which was chromatographed (on silica gel, eluted withmethylene chloride) to provide the main fraction as a pale tan solidweighing 1.48 g (62%). Recrystallization from ethyl acetate provided ananalytically pure sample as colorless fine matted needles, m. pt.185.5°-186° C.

EXAMPLES 4-7: Preparation and use of deprotonation imaging media

The following indicator dyes are used in the Examples below:

Dye A: 2-[2-octyloxyphenyl]-4-[4-dimethylaminophenyl]-6-phenylpyridine(available from Hilton Davis Co.)

Dye B: 2,4,6-tris[4-methoxyphenyl]pyridine (prepared in Example 1 above)

Dye C: 2,4,6-tris[2,4-dimethoxyphenyl]pyridine (prepared in Example 2above)

Dye D: 2,6-bis[4-methoxyphenyl]-4-[thienyl]pyridine (prepared by themethod of Example 1 above)

Dye E: 2,6-bis[4-methoxyphenyl]-4-[4-bromophenyl]pyridine (prepared bythe method of Example 1 above)

Dye F: 2,6-bis[4-methoxyphenyl]-4-[2-naphthyl]pyridine (prepared by themethod of Example 1 above)

Dye G: 2,4-bis[4-methoxyphenyl]-6-[2-naphthyl]pyridine (prepared by themethod of Example 3 above)

Dye H: 2,4,6-tris[2,4,6-trimethoxyphenyl]pyridine (prepared by themethod of Example 1 above)

Dye I: 2-[2-[2,4-bis[octyloxy]phenyl]ethen-1-yl]quinoline (availablefrom Yamada Chemical Co., Kyoto Japan)

Dye J: 2-[2-[4-dodecyloxy-3-methoxyphenyl]ethen-1-yl]quinoline(available from Yamada Chemical Co.)

Dye K:3',6'-bis[N-[2-chlorophenyl]-N-methylamino]spiro[2-butyl-1,1-dioxo[1,2-benzisothiazole-3(3H),9'-(9H)xanthene]](preparedby the method of U.S. Pat. No. 4,345,017)

Dye L:3',6'-bis[N-[2-[methanesulfonyl]phenyl]-N-methylamino]spiro[2-butyl-1,1-dioxo[1,2-benzisothiazole-3(3H),9'-(9H)xanthene]](preparedby the method of U.S. Pat. No. 4,345,017)

Dye M:9-diethylamino[spiro[12H-benzo(a)xanthene-12,1'(3'H)-isobenzofuran)-3'-one](available from Hilton Davis Co.)

Dye N:2'-di(phenylmethyl)amino-6'-[diethylamino]spiro[isobenzofuran-1(3H),9'-(9H)xanthen]-3-one(available from Hilton Davis Co.)

Dye O:3-[butyl-2-methylindol-3-yl]-3-[1-octyl-2-methylindol-3-yl]-1-(3H)-isobenzofuranone(available from Hilton Davis Co.)

Dye P:6-[dimethylamino]-3,3-bis[4-dimethylamino]-phenyl-(3H)-isobenzofuranone(available from Hilton Davis Co.)

EXAMPLE 4

This example illustrates the quantum yields of acid generation which maybe achieved using indicator sensitizing dyes in their protonated forms.

The general method for measurement of quantum yields involved coatingthe sensitizing indicator dye, a superacid precursor, and optionallyanother acid indicator dye, in a polymeric binder. In some experiments,a non-basic, polycyclic aromatic sensitizer (for example2-t-butylanthracene) was also present in the film to facilitategeneration of an initial amount of acid used to protonate thesensitizing indicator dye when the film is initially exposed toultra-violet radiation, as described below. The film was then irradiatednear the long wavelength absorption maximum of the protonatedsensitizing indicator dye. The absorbance at the wavelength of maximumabsorption due to the protonated sensitizing indicator dye was measuredbefore and after this exposure. From the change in absorption, andmeasurement of the exposure received by the sample, the quantum yield ofacid production could be calculated as described below.

By way of example, Dye A was tested in the following manner. A coatingfluid was prepared from Dye A (5.0 mg),[4-[2-hydroxytetradec-1-yloxy]phenyl]iodonium hexafluoroantimonate (5.0mg; this superacid precursor was used for all the quantum yielddeterminations) and 0.5 mL of a 7.5% (w/v) solution of polystyrene(molecular weight 45,000, available from Aldrich Chemical Co.,Milwaukee, Wis.) in 2-butanone. This solution was coated ontotransparent poly(ethylene terephthalate) film base 4 mil (101 μm) inthickness (ICI Type 3295, supplied by ICI Americas, Inc., Wilmington,Del.) using a #18 wire-wound coating rod. After drying overnight (underreduced pressure, in the dark, at room temperature), a portion of thefilm was briefly exposed to ultra-violet radiation from a low-powermercury lamp sufficient to generate enough acid in the film to partiallyprotonate the Dye A (the absorbance achieved at 460 nm was 0.16). Thefilm was then exposed to light at 460 nm (0.48 mW/cm²) for 60 seconds,using the output of a 150 W Xenon lamp, which had been spatiallyhomogenized by passing it through about 5-7 m of single core opticalfiber, and then passed through a three-cavity interference filter with abandpass of about 10 nm to select the wavelength. The light intensity atthe sample was measured with a calibrated photodiode. After exposure,the absorbance of the film at 456 nm (the wavelength of maximumabsorption of protonated Dye A) was measured and found to have increasedby 0.205 absorbance units due to acid generation during exposure, whichprotonated more Dye A. The quantum yield of acid photogeneration underthese exposure conditions was 0.13.

In this experiment, the protonated Dye A served as a photosensitizer ofthe superacid precursor, and the unprotonated Dye A in the film servedas an acid indicator dye, since its increased absorbance at 456 nm uponprotonation (ε⁸⁰ =33,000M⁻¹ cm⁻¹) is used to quantify the acid formed.

The amount of acid generated (Δn, in moles) in a given area of the film,is: ##EQU1## where a_(ex) is the area of the film exposed (in cm²),ΔA.sup.λ is the change in the absorbance of the film at the absorbancemaximum of the protonated indicator dye (or, if present, the alternativeacid indicator dye), and e.sup.λ is the molar absorptivity of theprotonated sensitizer or indicator dye at that wavelength in L mol⁻¹cm⁻¹.

This calculation assumes that every photogenerated proton protonates anunprotonated sensitizing indicator dye or alternative acid indicator dyemolecule, which will be the case if (1) the concentration of thesensitizing indicator dye or alternative acid indicator dye is large;(2) the sensitizing dye or alternative acid indicator dye is more basicthan other major components of the film; and (3) there is relativelylittle of any basic impurity in the sample (which would act as a basethreshold, to "buffer" the sample). Even if these conditions do not allhold, the calculation can still give valid relative results for similarsamples containing, for example, different sensitizing indicator dyes orsuperacid precursors.

Quantum yields of acid photogeneration (Φ_(acid)) are calculatedaccording to the relation:

    Φ.sub.acid =Δn/ζ.sub.abs

where ζ_(abs) is the number of photons absorbed by the sensitizer, ineinsteins (moles of photons). ζ_(abs) is determined by the relation:

    ζ.sub.abs =8.359×10.sup.-12 [Pt.sub.ex λ.sub.ex a.sub.ex (1-10.sup.-A.sbsp.av)] einstein.sup.-1 nm.sup.-1 mW.sup.-1 sec.sup.-1

where P is the incident (monochromatic or quasi-monochromatic) lightpower in mW cm⁻², t_(ex) is the duration of the irradiation in seconds,λ_(ex) is the exposure wavelength in nm, and A_(av) is the average (overthe exposure period) absorbance of the sample at the exposurewavelength. The term in parentheses is the average absorptance of thesample. In performing the measurement, care must be taken that (1) thearea of the film probed by the spectrophotometer light is fully withinthe exposed area; and (2) that exactly the same area of the film isprobed by the spectrophotometer before and after the exposure (becausethe films which are hand-coated are generally not very uniform inthickness).

The efficiency of the sensitization of superacid precursor decompositionis strongly dependent on the extent of the reaction, with thesensitization yield highest initially and decreasing as the exposureproceeds.

The quantum yields obtained are shown in Table 3 below, in which λ_(U)denotes the longest unprotonated absorption wavelength, λ_(P) denotesthe longest protonated absorption wavelength, λ_(IRR) denotes theirradiation wavelength and QY denotes the quantum yield.

                  TABLE 3                                                         ______________________________________                                        Dye  Class of dye                                                                              λ.sub.U, nm                                                                     λ.sub.P, nm                                                                   λ.sub.IRR, nm                                                                 QY, %                                 ______________________________________                                        A    Triarylpyridine                                                                           334      458    460    6.8                                   B    Triarylpyridine                                                                           330      370    400    7.8                                   C    Triarylpyridine                                                                           320      382    400    11.1                                  D    Triarylpyridine                                                                           340      372    400    4.8                                   E    Triarylpyridine                                                                           332      388    400    7.0                                   F    Triarylpyridine                                                                           332      386    400    5.5                                   G    Triarylpyridine                                                                           <320     374    400    6.2                                   H    Triarylpyridine                                                                           <320     366    400    3.5                                   I    Styrylquinoline                                                                           368      450    460    1.9                                   J    Styrylquinoline                                                                           366      438    460    2.3                                   K    Xanthene    358      554    560    0.23                                  L    Xanthene    356      552    544    0.13                                  M    Xanthene    356      528    560    0.21                                  N    Fluoran     380      582    580    0.16                                  O    Phthalide   358      536    544    0.18                                  P    Phthalide   358      616    620    0.065                                 ______________________________________                                    

EXAMPLE 5

This example illustrates the use of a triarylpyridine indicatorsensitizing dye which is coated in its protonated form as thehexafluoroantimonate salt. The protonated indicator dye is used tosensitize an imaging medium of the present invention to nearultra-violet exposure, to produce a final image having, in D_(max)areas, significant absorption in the near ultra-violet and visibleregions but, in D_(min) areas, negligible absorption in either the nearultra-violet or visible regions.

Two coating fluids were prepared as follows:

Fluid A

An indicator sensitizing dye (2,4,6-tris[2,4-dimethoxyphenyl]pyridiniumhexafluoroantimonate, prepared by washing a dichloromethane solution ofDye C with aqueous hexafluoroantimonic acid, then separating and dryingthe dichloromethane layer, 10 mg), a superacid precursor([4-octyloxyphenyl]phenyliodonium hexafluoroantimonate, 20 mg, preparedas described in U.S. Pat. No. 4,992,571), a secondary acid generator(2,2-dimethyl-1-[4-benzyloxybenzyloxalyloxy]prop-3-yl[4-benzyloxybenzyl]oxalate(prepared as described in the aforementioned copending application Ser.No. 08/141,852, 80 mg) and the polystyrene binder as in Example 4 (800mg of a 20% solution in 2-butanone) were combined together and heated atabout 50° C. until complete dissolution of all components had beenachieved.

Fluid B

A dispersion was first prepared as follows. A magenta indicator dye(3-[butyl-2-methylindol-3-yl]-3-[1-octyl-2-methylindol-3-yl]-1-(3H)-isobenzofuranone,available from Hilton Davis Co. under the tradename Copikem 35, 1.0 g)and an acrylate polymeric binder (Acryloid B-82, supplied by Rohm andHass Corporation, Philadelphia, Pa. 19104, 1.25 g) were dissolved inethyl acetate (10 g), and the resultant solution was added to 7 g of a2.55% solution of poly(vinyl alcohol) (Vinol 540, available from AirProducts Corporation, Allentown, Pa.) in water. The mixture was thensonicated, after which evaporation of ethyl acetate afforded therequired dispersion (14.5% solids by weight).

To 1.0 g of this dispersion was added 100 mg of a 20% aqueous solutionof potassium acetate and 10 mg of a 5% aqueous solution of a surfactant(Igepal CO-630 (available from GAF Corporation, 1361 Alps Road, WayneN.J. 07470) to form Fluid B.

Fluid A was coated onto transparent poly(ethylene terephthalate) base 4mil (101 μm) in thickness (ICI Type 3295, supplied by ICI Americas,Inc., Wilmington, Del.) using a #4 coating rod, to form anacid-generating layer, and the UV/visible absorption spectrum of thiscoating was measured using a piece of the uncoated film base as thereference. Fluid B was next coated on top of the acid-generating layerusing a #3 coating rod, to form a color-change layer.

A portion of imaging medium so produced was exposed to ultravioletradiation at an exposure of 4.3 mJ/cm² using a nuArc 26-1K MercuryExposure System (available from nuArc Company, Inc., 6200 West HowardStreet, Niles, Ill. 60648; the output of this device was measured usingan IL390A "Light Bug" radiometer, available from International Light,Inc., 17 Graf Road, Newburyport, Mass. 01950); after this exposure theentire imaging medium was heated first at 45° C. for 20 seconds and thenat 120° C. for 30 seconds. The UV/visible absorption spectrum of theexposed and unexposed regions were then measured, again using a piece ofuncoated film base as the reference.

Table 4 below shows the absorbances at one wavelength in the nearultra-violet and one wavelength in the visible region at the variousstages of imaging, with uncoated film base as the reference.

                  TABLE 4                                                         ______________________________________                                                  Acid-generating         Non-exposed                                 Absorption                                                                              layer before                                                                              Exposed area                                                                              area after                                  wavelength, nm                                                                          imaging     after heating                                                                             heating                                     ______________________________________                                        366       0.312       0.468       0.023                                       544       0.005       1.105       0.004                                       ______________________________________                                    

As can be seen in Table 4, the absorbance in the near ultra-violet at366 nm in the D_(min) region of the imaged film is 0.023, whereas beforeimaging the absorbance at this wavelength (due to protonated Dye C) was0.312. On the other hand, in the D_(max) region the absorbance at thiswavelength is 0.468, due partly to protonated Dye C and partly to theultra-violet absorbance of the magenta image dye, whose maximumabsorbance is at 544 nm (with an absorbance of 1.105).

EXAMPLE 6

This example illustrates the use, in a process of the present inventionsimilar to that described above with reference to Table 1, of a styrylquinoline indicator sensitizer dye which is coated in its unprotonatedform. A non-basic, polycyclic aromatic sensitizer which absorbssignificantly only below 370 nm (in the near ultra-violet) is alsocoated. The protonated form of the indicator sensitizer dye absorbs inthe blue visible region. The protonated form of the image dye usedabsorbs in the green region of the visible spectrum. The imaging mediumused is first blanket exposed to near ultra-violet radiation, generatingacid from the non-basic polycyclic aromatic sensitizer and the superacidprecursor which protonates the indicator sensitizing dye. The medium isthen imagewise exposed to blue light, which is absorbed by theprotonated form of the indicator sensitizing dye. After heating, a finalimage having significant absorption in the green region of the spectrumin D_(max) areas but negligible absorption in either the blue or greenregions of the spectrum in D_(min) areas is obtained.

Two coating fluids were prepared as follows:

Fluid A

Indicator sensitizing dye Dye I (1.5 mg), a non-basic polycyclicaromatic sensitizer 1-[prop-1-enyl]pyrene (10 mg), a secondary acidgenerator1-[4-benzyloxybenzyloxalyloxy]oct-2-yl[4-benzyloxybenzyl]oxalate(prepared according to the methods described in the aforementionedcopending application Ser. No. 08/141,852, 60 mg) and the same superacidprecursor (15 mg) and polystyrene binder (600 mg of a 20% solution in2-butanone) as in Example 5 above were combined and heated at about 50°C. until complete dissolution of all components had been achieved.

Fluid B

To 1.0 g of the dispersion prepared by the method described in Example 5above (17% solids) was added 100 mg of a 2% aqueous solution ofpotassium acetate and 50 mg of a 5% aqueous solution of surfactantIgepal CO-630.

Fluid A was coated onto reflective Melinex base 4 mil (101 μm) inthickness using a #4 coating rod, to form an acid-generating layer.Fluid B was coated on top of this layer using a #3 coating rod, to forma color-change layer.

The entire imaging medium so produced was exposed to ultravioletradiation using 4.5 "Units" of exposure from the same nuArc apparatus asin Example 5 above filtered through a U360 colored-glass bandpass filter(available from Hoya Corporation, Fremont, Calif.). The exposurereceived by the film was measured as 7.6 mJ/cm² using the sameradiometer as in Example 5 above. After this exposure, a portion of theimaging medium was exposed to filtered radiation for various lengths oftime from a 150 W Xenon lamp, using the apparatus described in Example 4above with a three-cavity interference filter having a bandpass of about10 nm centered at 440 nm. After this Xenon lamp exposure, the wholeimaging medium was heated first at 45° C. for 20 seconds and then at120° C. for 30 seconds. The blue and green reflection optical densitieswere measured at each stage of this process using an X-Rite 310photographic densitometer, supplied by X-Rite, Inc., Grandville, Mich.,equipped with the appropriate filter. The results obtained are shown inTable 5 below.

                  TABLE 5                                                         ______________________________________                                        Exposure                                                                             Exposure at                                                                              Heating time,                                               at 360 nm                                                                            440 nm     sec. at 45° C.                                                                    Blue OD Green OD                                 (mJ/cm.sup.2)                                                                        (mJ/cm.sup.2)                                                                            (90° C.)                                                                          (reflection)                                                                          (reflection)                             ______________________________________                                        0      0          0 (0)      0.06    0.04                                     7.6    0          0 (0)      0.21    0.04                                     7.6    0          20 (30)    0.07    0.10                                     7.6    40         20 (30)    0.38    0.79                                     7.6    60         20 (30)    0.49    1.86                                     7.6    80         20 (30)    0.63    2.59                                     ______________________________________                                    

As can be seen from Table 5, after exposure at 360 nm the opticaldensity in the blue region was 0.21, meaning that about 38% of incidentblue light was absorbed by protonated Dye I. The imaging medium couldthen be imaged using blue light to give a green D_(max) of 2.59 byprotonation of the image dye. However, D_(min) regions of the imagedfilm exhibited optical density in the blue region of only 0.07, showingthat Dye I had been returned to the unprotonated form in which it wasoriginally coated (when it gave a blue optical density of 0.06).

EXAMPLE 7

This example illustrates the use of a xanthene indicator sensitizing dyewhich is coated in its unprotonated form. A non-basic, polycyclicaromatic sensitizer which absorbs significantly only below 370 nm (inthe near ultra-violet) is also coated. The protonated form of theindicator sensitizing dye absorbs in the blue visible region, while theprotonated form of the image dye used also absorbs in the green regionof the spectrum. The imaging medium is first blanket exposed to nearultra-violet radiation, generating acid from the polycyclic aromaticsensitizer and the superacid precursor and protonating the indicatorsensitizing dye. The medium is then imagewise exposed to green light,which is absorbed by the protonated form of the indicator sensitizingdye. After heating, a final image having significant absorption in thegreen region of the spectrum in D_(max), areas but negligible absorptionin the green region of the spectrum in D_(min) areas is obtained.

Two coating fluids were prepared as follows:

Fluid A

Indicator sensitizing dye Dye K (2.4 mg), a non-basic polycyclicaromatic sensitizer (1-[prop-1-enyl]pyrene, 15 mg), and the samesuperacid precursor (30 mg), secondary acid generator (120 mg) andpolystyrene binder (1.2 g of a 20% solution in 2-butanone) as in Example5 above were combined and heated at about 50° C. until completedissolution of all components had been achieved. The same Fluid B as inExample 6 above was used.

Fluid A was coated onto reflective Melinex base 4 mil (101 μm) inthickness using a #4 coating rod, to form an acid-generating layer.Fluid B was coated on top of this layer using a #3 coating rod, to forma color-change layer.

The entire imaging medium so produced was exposed to ultravioletradiation in the same way as in Example 6 above using 3 "Units" ofexposure. The exposure received by the film under these conditions wasmeasured (using the same radiometer as in Example 5) to be 4.1 mJ/cm²) .After this exposure, a portion of the imaging medium was exposed tofiltered radiation for various lengths of time from a 150 W Xenon lamp,using the apparatus described in Example 4 above with a three-cavityinterference filter having a bandpass of about 10 nm centered at 560 nm.After this exposure, the whole imaging medium was heated first at 45° C.for 20 seconds and then at 120° C. for 30 seconds. The green reflectionoptical density was measured at each stage of this process using thesame densitometer as in Example 6. The results obtained are shown inTable 6 below

                  TABLE 6                                                         ______________________________________                                        Exposure at                                                                            Exposure at  Heating time,                                           360 nm   560 nm       sec. at 45° C.                                                                    Green OD                                     (mJ/cm.sup.2)                                                                          (mJ/cm.sup.2)                                                                              (90° C.)                                                                          (reflection)                                 ______________________________________                                        0        0            0 (0)      0.06                                         4.1      0            0 (0)      0.34                                         4.1      0            20 (30)    0.09                                         4.1      270          20 (30)    0.93                                         4.1      540          20 (30)    2.48                                         ______________________________________                                    

As can be seen from Table 6, after exposure at 360 nm the opticaldensity in the green region was 0.34, meaning that about 54% of incidentgreen light was absorbed by protonated Dye K. The imaging medium couldthen be imaged using green light to give a green D_(max) of 2.59 byprotonation of the image dye. However, D_(min) regions of the imagedfilm exhibited optical density in the green region of only 0.09, showingthat Dye K had been substantially returned to the unprotonated form inwhich it was originally coated (when it gave a green optical density of0.06).

EXAMPLES 8-11: Nucleophile Processes EXAMPLES 8-9: Preparation ofsensitizing dyes for nucleophile processes EXAMPLE 8: Preparation of1-methyl-2-[4-diphenylaminophenyl]ethen-1-yl]quinoliniumhexafluoroantimonate

A mixture of quinaldinium methiodide (713 mg, 2.5 mmole),diphenylbenzaldehyde (683 mg, 2.3 mmole) and acetic anhydride (8 mL) wasstirred with heating to reflux for 15 minutes, then cooled toapproximately 100° C. and maintained at that temperature for a further90 minutes. The reaction mixture was then cooled to 20° C., whereupon asmall amount of solid precipitated. The reaction mixture was thendiluted with acetone (8 mL) and diethyl ether (20 mL) and filtered toprovide 665 mg of brick-red prisms. A 600 mg sample of this crudeproduct was chromatographed on silica gel, eluting with a 5%methanol/dichloromethane mixture, to provide 84.5 mg of the desired pureiodide salt as a red-brown amorphous solid. This solid was dissolved inmethanol (8 mL) and stirred with 72 mg (0.2 mmole) of silverhexafluoroantimonate dissolved in methanol (0.7 mL). The resultingprecipitate was filtered off and the filtrate diluted with a solution ofsodium hexafluoroantimonate (0.5 g) in water (30 mL). The precipitatewhich resulted was filtered off and dried in vacuo at 25° C. overnightto give the desired hexafluoroantimonate salt as brick-red prisms (41mg), having λ_(max) (CH₂ Cl₂) 550 μm, ε=35,430. A dichloromethanesolution of the product was 67% bleached after two minutes andcompletely bleached within ten minutes by addition of the nucleophilicamine 3-aminopropanol.

EXAMPLE 9: Preparation of3,3-dimethyl-1-methyl-2-[2-[9-phenylcarbazol-3-yl]ethen-1-yl]-3H-indoliumhexafluoroantimonate

A mixture of fleshly-distilled Fischer's base (0.70 g, 4.0 mmole) andN-phenylcarbazole-3-carboxaldehyde (1.0 g g, 4.0 mmole) in toluene (3mL) was heated to approximately 50° C., then phosphorus oxychloride (0.3mL) was added, resulting in a vigorous (almost violent) exotherm. Theresultant mixture was then stirred at ambient temperature forapproximately 30 minutes, water (15 mL) was added, and the supernatantwas decanted and discarded. The glassy residue remaining was taken up inwarm methanol (7 mL) and quenched into water (60 mL) to give a brick-redprecipitate, which was collected by filtration, then chromatographed onsilica gel, eluting with a 5% methanol/dichloromethane mixture, followedby a 10% methanol/dichloromethane mixture. The purest fractions from thechromatography were combined and evaporated to dryness, then the solidresidue was taken up in dichloromethane (12 mL) and washed with 10%hexafluoroantimonic acid (2×4 mL aliquots), then with water (6 mL), andfinally evaporated to a brittle foam (1.15 g). A 250 mg sample of thisfoam was recrystallized from chloroform (1.5 mL) to provide the desiredpure product (206 mg) as orange-red prisms, having λ_(max) (CH₂ Cl₂) 512nm, ε=46,170. A dichloromethane solution of the product was completelybleached within one minute by addition of the nucleophilic amine3-aminopropanol.

EXAMPLES 10-11: Preparation and use of nucleophile imaging media

The following nucleophile bleachable sensitizing dyes are used in theExamples below:

Dye Q: 1-methyl-2-[2-[2,4-bis[octyloxy]phenyl]ethen-1-yl]quinoliniumhexafluoroantimonate (available from methylation of Dye I above)

Dye R: 1-methyl-2-[4-diphenylaminophenyl]ethen-1-yl]quinoliniumhexafluoroantimonate (prepared in Example 8 above)

Dye S:3,3-dimethyl-1-methyl-2-[2-[9-phenylcarbazol-3-yl]ethen-1-yl]-3H-indoliumhexafluoroantimonate (prepared in Example 9 above)

Dye T:3,3-dimethyl-1-methyl-2-[2-[9-ethylcarbazol-3-yl]ethen-1-yl]-3H-indoliumhexafluoroantimonate (prepared using a method analogous to thatdescribed in Example 9 above)

The following image dye was used in Example 10 below:

Dye U:3',6'-bis[2,3-dihydro-2,3,3-trimethylindol-1-yl]spiro[2-[2-methylprop-1-yl]-1,1-dioxo[1,2-benzisothiazole-3(3H),9'(9H)xanthene]](prepared in a manner analogous to that described in U.S. Pat. No.4,345,017).

EXAMPLE 10

This Example illustrates the quantum yields of acid generation which maybe achieved using hemicyanine dyes in their first form, such dyes beingcapable of being converted to a second form, which does not absorbvisible light, by addition of a nucleophile.

The general method for measurement of quantum yields was very similar tothat described in Example 4 above. However, in that Example theformation of acid could be measured by conversion of the unprotonatedsensitizing dye itself to its protonated form. The hemicyanine dyesdescribed in this Example do not readily undergo protonation, and in anycase such protonation would not have a predictable effect on the dye'sabsorption characteristics. Therefore, a separate acid indicator (image)dye was incorporated into the test film with the hemicyanine sensitizingdye and the superacid precursor, and the change in absorption of thisimage dye, due to protonation, was used to quantify the amount of acidproduced from irradiation of the sensitizing dye. The absorptionspectrum of the unprotonated and protonated forms of this image dye wereselected so as to overlap minimally with the absorption due to thehemicyanine sensitizer.

Two different indicator dyes were used in these experiments: Dye A andDye U. Their extinction coefficients were taken to be, respectively,33,000 M⁻¹ cm⁻¹ and 98,000 M⁻¹ cm⁻¹.

By way of Example, Dye Q was tested in the following manner. A coatingfluid was prepared from Dye Q (3 mg), the same superacid precursor as inExample 4 above (5 mg), Dye U (5 mg) and the same polystyrene as inExample 4 (0.5 mL of a 7.5% w/v solution in 2-butanone). This coatingfluid was coated on to the same transparent film base as described inExample 4. The resulting acid-generating layer was not exposed toultraviolet light, as in Example 4, but instead was directly exposed at501 nm. After exposure, the layer was heated at 81° C. for 40 seconds,and the change in absorption due to the indicator dye, Dye U, wasmeasured at 656 nm. All of the acid produced photochemically was assumedto have protonated dye U, and therefore the absorption change of the dyeU was used in the calculation described in Example 4 to obtain thequantum yield of acid formation. Dye U was used as the image dye, exceptin the case of Dye R, for which Dye A was used as the image dye.

In addition to the assumptions mentioned in Example 4, it is alsoassumed that the indicator dye (Dye A or Dye U) is photochemicallyinert. However, it is likely that these dyes may function to some extentas cosensitizers, and the quantum yield measured in a different systemmay be different from that quoted here.

The quantum yields obtained are shown in Table 7 below, in which λ₁denotes the absorption maximum of the sensitizing dye in the film, λ₂denotes the irradiation wavelength, and QY denotes the quantum.

                  TABLE 7                                                         ______________________________________                                        Dye     λ.sub.1 (nm)                                                                         λ.sub.2 (nm)                                                                    QY (%)                                         ______________________________________                                        Q       450           501      0.9                                            R       530           544      1.4                                            S       488           501      0.65                                           T       500           501      0.53                                           ______________________________________                                    

EXAMPLE 10

This Example illustrates the use of a hemicyanine sensitizing dye (DyeS), with and without added cosensitizer, to form an image by anucleophile process of the present invention. The color-change layerused in this Example contains a nucleophilic primary amine base, whichdecolorizes the sensitizing dye when the film is heated after imaging.

A stock solution was prepared by dissolving the same superacid precursoras in Example 4 above (40 mg), the same secondary acid generator as inExample 6 above (120 mg) and Dye S (20 mg) in a solution of the samepolystyrene as in Example 4 (4 g of a 10% w/w solution in 2-butanone).Coating fluids A-F were prepared by adding, to 400 mg of this stocksolution, the following various cosensitizers:

    ______________________________________                                        Fluid A     No cosensitizer (control)                                         Fluid B     5 mg triphenylamine                                               Fluid C     9.6 mg tris(p-bromophenyl)amine                                   Fluid D     5.4 mg N-(p-methoxyphenyl)carbazole                               Fluid E     4.8 mg N-phenylcarbazole                                          Fluid F     4.4 mg 2,5-di-t-butylhydroquinone.                                ______________________________________                                    

These amounts of cosensitizers were chosen so that the molar ratio ofcosensitizer to superacid precursor in the coating solutions wasapproximately 3.7.

Coating fluids A-F were coated on to reflective Melinex film base of 5mil (126 μm) thickness using a #8 coating rod to form an acid-generatinglayer. The resultant films were overcoated to form a color-change layerusing a #12 coating rod with a dispersion prepared as follows.

The same Copikem 35 indicator dye as in Example 5 above (2.5 g) and anamine base (1-(3-aminoprop-1-yl)imidazole, 0.275 g) were added to asolution of the same Acryloid B-82 binder as in Example 5 (7.5 g in 40 gof ethyl acetate), and the resultant solution was added to a solution ofthe same Vinol 540 as used in Example 5 (28.6 g of a 7% solution inwater). Water (55 g) was added, and the resultant mixture was sonicated.Evaporation of ethyl acetate afforded the required aqueous dispersion(61.2 g, 16.9% solids by weight).

The resultant imaging media were exposed to light at 501 nm (0.93 mW/cm²exposure) through a transmission step wedge for 1800 seconds. Afterexposure, the films were heated at 60° C. for 30 seconds, then at 120°C. for 40 seconds. Optical densities (green) were recorded in D_(min)(non-exposed) and D_(max) (exposed) areas using the same densitometer asin Example 6 with the appropriate filter (Status A). Table 8 showsoptical densities before imaging and after exposure and heating, theminimum exposure required to attain D_(max), and the oxidation potentialof the cosensitizer (measured in acetonitrile solution against astandard calomel electrode).

                  TABLE 8                                                         ______________________________________                                               OD       D.sub.max                                                                              D.sub.min                                                                            Energy to                                                                             Oxidation                                    before   after    after  D.sub.max                                                                             Potential                             Medium imaging  imaging  imaging                                                                              (mJ/cm.sup.2)                                                                         (mV)                                  ______________________________________                                        A      0.7      --       0.10   >1674    1410*                                B      0.71     0.65     0.08   15.9     956                                  C      0.69     0.69     0.10   82      1109                                  D      0.72     0.64     0.09   293     1260                                  E      0.70     --       0.10   >1674   1312                                  F      0.74     0.68     0.10   293     Un-                                                                           known                                 ______________________________________                                         *Since no cosensitizer was added, the oxidation potential quoted is that      of the hemicyanine dye itself.                                           

From the data in Table 8, it will be seen that in all cases thesensitizing dye was bleached after imaging, as shown by a drop inoptical density from about 0.7 before heating to about 0.1 afterheating. The sensitivity of the medium, as measured by the energyrequired to reach D_(max) was highly dependent upon the cosensitizerused; without a cosensitizer, the medium could not be imaged at all withless than 1674 mJ/cm² exposure, whereas with triphenylamine ascosensitizer, D_(max) was attained at an exposure of 15.9 mJ/cm².Furthermore, the effectiveness of a cosensitizer was inversely relatedto its oxidation potential

From the foregoing, it will be seen that the present invention providesan imaging medium, and a process for producing an image, which overcomesthe imitations of the '489 and '612 processes. In particular, thepresent invention allows imagewise exposure to be effected usingradiation in the same wavelength range as that in which the image is tobe viewed, and also enables an image to be produced in which the D_(min)regions do not exhibit the absorption of the sensitizing dye used in theimaging process. Also, certain processes of the present invention canproduce a full color image using imagewise exposures at only onewavelength, with blanket exposures at two further wavelengths.

We claim:
 1. An imaging medium comprising an acid-generating layer orphase comprising a mixture of a superacid precursor, a sensitizing dyeand a secondary acid generator, and a color-change layer or phasecomprising an image dye;the sensitizing dye having a first form and asecond form, the first form having substantially greater substantialabsorption in the first wavelength range than the second form; thesuperacid precursor being capable of being decomposed to producesuperacid by actinic radiation in a second wavelength range differentfrom the first wavelength range, but not, in the absence of thesensitizing dye, being capable of being decomposed to produce superacidby actinic radiation in the first wavelength range; the secondary acidgenerator being capable of thermal decomposition to form a secondaryacid, the thermal decomposition of the secondary acid generator beingcatalyzed by unbuffered superacid; and the image dye undergoing a changein its absorption of radiation upon contact with the secondary acid theimaging medium further comprising a reagent capable of converting thesensitizing dye from its first form to its second form, said reagentbeing present either (a) in the color-change layer or phase; or (b) in athird layer interposed between the acid-generating layer and thecolor-change layer.
 2. An imaging medium according to claim 1 whereinthe sensitizing dye can be converted from its first form to its secondform by contact with a base.
 3. An imaging medium according to claim 2wherein the sensitizing dye is selected from the group consisting offluoran dyes, phthalide dyes, xanthene dyes, acridine dyes, andsubstituted quinoline and pyridine dyes.
 4. An imaging medium accordingto claim 3 wherein the sensitizing dye is a triarylpyridinium dye.
 5. Animaging medium according to claim 1 wherein the first form of thesensitizing dye is a protonated form and the second form is adeprotonated form, the two forms being reversibly interconverted bycontact with base or acid.
 6. An imaging medium according to claim 5comprising:a first acid-generating layer comprising a sensitizing dye inits protonated form, a superacid precursor and a secondary acidgenerator; a first color-change layer disposed adjacent the firstacid-generating layer and comprising a base and a first image dyeundergoing a change in its absorption of radiation upon contact with thesecondary acid generated upon acid-catalyzed decomposition of thesecondary acid generator in the first acid-generating layer; a firstacid-resistant interlayer superposed on the first acid-generating layerand the first color-change layer; a second acid-generating layerdisposed on the opposed side of the first acid-resistant interlayer fromthe first acid-generating layer and the first color-change layer, thesecond acid-generating layer comprising a sensitizing dye in itsunprotonated form, a superacid precursor and a secondary acid generator,the second acid-generating layer further comprising a first auxiliarysensitizer which renders the superacid precursor therein susceptible todecomposition by actinic radiation of a first wavelength in the secondwavelength range, but not susceptible to decomposition by actinicradiation of a second wavelength in the second wavelength range; asecond color-change layer disposed adjacent the second acid-generatinglayer and on the opposed side of the first acid-resistant interlayerfrom the first acid-generating layer and the first color-change layer,the second color-change layer comprising a base and a second image dyeundergoing a change in its absorption of radiation upon contact with thesecondary acid generated upon acid-catalyzed decomposition of thesecondary acid generator in the second acid-generating layer, theabsorption change undergone by the second image dye being different fromthat undergone by the first image dye; a second acid-resistantinterlayer disposed on the opposed side of the second acid-generatinglayer and second color-change layer from the first acid-resistantinterlayer; a third acid-generating layer disposed on the opposed sideof the second acid-resistant interlayer from the second acid-generatinglayer and second color-change layer, the third acid-generating layercomprising a sensitizing dye in its unprotonated form, a superacidprecursor and a secondary acid generator, the third acid-generatinglayer further comprising a second auxiliary sensitizer which renders thesuperacid precursor therein susceptible to decomposition by actinicradiation of the second wavelength in the second wavelength range, butnot susceptible to decomposition by actinic radiation of the firstwavelength in the second wavelength range; and a third color-changelayer disposed adjacent the third acid-generating layer and on theopposed side of the second acid-resistant interlayer from the secondacid-generating layer and the second color-change layer, the thirdcolor-change layer comprising a base and a third image dye undergoing achange in its absorption of radiation upon contact with the secondaryacid generated upon acid-catalyzed decomposition of the secondary acidgenerator in the third acid-generating layer, the absorption changeundergone by the third image dye being different from those undergone bythe first and second image dyes.
 7. An imaging medium according to claim6 wherein the same sensitizing dye, superacid precursor and secondaryacid generator are present in each of the three acid-generating layers.8. An imaging medium according to claim 1 wherein the conversion of thesensitizing dye from its first form to its second form is effected by anessentially irreversible chemical change.
 9. An imaging medium accordingto claim 8 wherein the conversion of the sensitizing dye from its firstform to its second form can be effected by contact the sensitizing dyewith a nucleophile.
 10. An imaging medium according to claim 9 whereinthe nucleophile is a primary or secondary amine.
 11. An imaging mediumaccording to claim 9 wherein the sensitizing dye is a hemicyanine dye.12. An imaging medium according to claim 11 wherein the sensitizing dyeis of the formula: ##STR13## wherein: G is a CR^(c) R^(d) group, aCR^(c) =CR^(d) group, an oxygen or sulfur atom, or an NR^(b) group;R^(a)and R^(b) are each an alkyl group containing from about 1 to about 20carbon atoms; R^(c) and R^(d) are each a hydrogen atom or an alkyl groupcontaining from about 1 to about 20 carbon atoms; n is 1 or 2; Ar is anaryl or heterocyclyl group; and X⁻ is an anion.
 13. An imaging mediumaccording to claim 12 wherein the sensitizing dye is any one or moreof:1-Methyl-2-[2-[2,4-bis[octyloxy]phenyl]ethen-1-yl]quinoliniumhexafluoroantimonate;1-Methyl-2-[4-diphenylaminophenyl]ethen-1-yl]quinoliniumhexafluoroantimonate;3,3-Dimethyl-1-methyl-2-[2-[9-phenylcarbazol-3-yl]ethen-1-yl]-3H-indoliumhexafluoroantimonate; and3,3-Dimethyl-1-methyl-2-[2-[9-ethylcarbazol-3-yl]ethen-1-yl]-3H-indoliumhexafluoroantimonate.
 14. An imaging medium according to claim 1 whereinthe layer or phase comprising the sensitizing dye further comprises acosensitizer which is a reducing agent less basic than the secondaryacid generator.
 15. An imaging medium according to claim 14 wherein thecosensitizer is a triarylamine or a hydroquinone.
 16. An imaging mediumaccording to claim 1 which is essentially anhydrous.
 17. An imagingmedium according to claim 1 wherein the acid-generating and color-changelayers or phases each comprise a polymeric binder.
 18. An imaging mediumaccording to claim 1 wherein the first form of the sensitizing dye hassubstantial absorption in the wavelength range of about 400 to about 900nm, but the second form has substantial absorption only below about 400nm, the difference between the wavelengths of maximum absorption of thefirst and second forms of the sensitizing dye being at least about 50nm.
 19. An imaging medium according to claim 1 wherein the first form ofthe sensitizing dye has an absorption peak in the visible range, and theimage dye, upon contact with the secondary acid, undergoes an absorptionchange visible to the human eye.
 20. An imaging medium according toclaim 1 wherein the first form of the sensitizing dye has substantialabsorption in the wavelength range of about 330 to about 450 nm, but thesecond form has substantial absorption only below about 330 nm, thedifference between the wavelengths of maximum absorption of the firstand second forms of the sensitizing dye being at least about 50 nm. 21.An imaging medium according to claim 1 wherein the first form of thesensitizing dye has an absorption peak in the range of 700-1200 nm, thesecond form of the sensitizing dye has a substantially lower visibleabsorption than the first from of the same dye, and the image dye, uponcontact with the seconday acid, undergoes an absorption change visibleto the human eye.
 22. An imaging medium according to claim 1 wherein thesuperacid precursor comprises an iodonium compound.
 23. An imagingmedium according to claim 22 wherein the superacid precursor comprises adiphenyliodonium compound.
 24. An imaging medium according to claim 1wherein the secondary acid generator is an oxalate or a3,4-disubstituted-cyclobut-3-ene-1,2-dione in which at least one of the3- and 4-substituents consists of an oxygen atom bonded to the squaricacid ring, and an alkyl or alkylene group, a partially hydrogenated arylor arylene group, or an aralkyl group bonded to said oxygen atom, said3,4-disubstituted-cyclobut-3-ene-1,2-dione being capable of decomposingso as to cause replacement of the or each original alkoxy, alkyleneoxy,aryloxy, aryleneoxy or aralkyloxy group of the derivative with ahydroxyl group, thereby producing squaric acid or an acidic squaric acidderivative having one hydroxyl group.
 25. An imaging medium according toclaim 24 wherein the 3,4-disubstituted-cyclobut-3-ene-1,2-dione isselected from the group consisting of:(a) primary and secondary estersof squaric acid in which the α-carbon atom bears a non-basiccation-stabilizing group; (b) tertiary esters of squaric acid in whichthe α-carbon atom does not have an sp² or sp hybridized carbon atomdirectly bonded thereto; and (c) tertiary esters of squaric acid inwhich the α-carbon atom does have an sp² or sp hybridized carbon atomdirectly bonded thereto, provided that this sp² or sp hybridized carbonatom, or at least one of these sp² or sp hybridized carbon atoms, ifmore than one such atom is bonded directly to the α-carbon atom, isconjugated with an electron-withdrawing group.
 26. An imaging mediumaccording to claim 24 wherein the3,4-disubstituted-cyclobut-3-ene-1,2-dione is of one of the followingformulae: ##STR14## in which R¹ is an alkyl group, a partiallyhydrogenated aromatic group, or an aralkyl group, and R² is a hydrogenatom or an alkyl, cycloalkyl, aralkyl, aryl, amino, acylamino,alkylamino, dialkylamino, alkylthio, alkylseleno, dialkylphosphino,dialkylphosphoxy or trialkylsilyl group, subject to the proviso thateither or both of the groups R¹ and R² may be attached to a polymer;##STR15## in which R¹ and R³ independently are each an alkyl group, apartially hydrogenated aryl group or an aralkyl group, subject to theproviso that either or both of the groups R¹ and R³ may be attached to apolymer; and ##STR16## in which n is 0 or 1, and R⁴ is an alkylene groupor a partially hydrogenated arylene group;or the squaric acid derivativecomprises at least one unit of the formula: ##STR17## in which n is 0 or1, and R⁵ is an alkylene or partially hydrogenated arylene group.
 27. Animaging medium according to claim 24 wherein wherein the oxalate isselected from the group consisting of:(a) primary and secondary estersof oxalic acid in which the α-carbon atom bears a non-basiccation-stabilizing group; (b) tertiary esters of oxalic acid in whichthe α-carbon atom does not have an sp² or sp hybridized carbon atomdirectly bonded thereto; (c) tertiary esters of oxalic acid in which theα-carbon atom does have an sp² or sp hybridized carbon atom directlybonded thereto, provided that this sp² or sp hybridized carbon atom, orat least one of these sp² or sp hybridized carbon atoms, if more thanone such atom is bonded directly to the α-carbon atom, is conjugatedwith an electron-withdrawing group; (d) an ester formed by condensationof two moles of an alcohol with the bis(hemioxalate) of a diol, providedthat the ester contains at least one ester grouping of type (a), (b) or(c); (e) polymeric oxalates derived from polymerization of oxalateesters having an ethylenically unsaturated group, provided that theester contains at least one ester grouping of type (a), (b) or (c); and(f) condensation polymers of oxalates, provided that the ester containsat least one ester grouping of type (a), (b) or (c) above.
 28. Animaging medium comprising an acid-generating layer or phase comprising amixture of a superacid precursor, a sensitizing dye and a secondary acidgenerator, and a color-change layer or phase comprising an image dye;thesensitizing dye having a first form and a second form, the first formhaving substantially greater substantial absorption in the firstwavelength range than the second form, the first form of the dye beingconvertible to its second form by contact with a base; the superacidprecursor being capable of being decomposed to produce superacid byactinic radiation in a second wavelength range different from the firstwavelength range, but not, in the absence of the sensitizing dye, beingcapable of being decomposed to produce superacid by actinic radiation inthe first wavelength range; the secondary acid generator being capableof thermal decomposition to form a secondary acid, the thermaldecomposition of the secondary acid generator being catalyzed byunbuffered superacid; and the image dye undergoing a change in itsabsorption of radiation upon contact with the secondary acid.
 29. Animaging medium comprising an acid-generating layer or phase comprising amixture of a superacid precursor, a sensitizing dye and a secondary acidgenerator, and a color-change layer or phase comprising an image dye;thesensitizing dye having a first form and a second form, the first formhaving substantially greater substantial absorption in the firstwavelength range than the second form, the first form of the dye beingconvertible to its second form by contact with a nucleophile; thesuperacid precursor being capable of being decomposed to producesuperacid by actinic radiation in a second wavelength range differentfrom the first wavelength range, but not, in the absence of thesensitizing dye, being capable of being decomposed to produce superacidby actinic radiation in the first wavelength range; the secondary acidgenerator being capable of thermal decomposition to form a secondaryacid, the thermal decomposition of the secondary acid generator beingcatalyzed by unbuffered superacid; and the image dye undergoing a changein its absorption of radiation upon contact with the secondary acid. 30.An imaging medium comprising an acid-generating layer or phasecomprising a mixture of a superacid precursor, a sensitizing dye and asecondary acid generator, and a color-change layer or phase comprisingan image dye;the sensitizing dye having a first form and a second form,the first form having substantially greater substantial absorption inthe first wavelength range than the second form, the first form of thedye being convertible to its second form by thermal decomposition of thedye; the superacid precursor being capable of being decomposed toproduce superacid by actinic radiation in a second wavelength rangedifferent from the first wavelength range, but not, in the absence ofthe sensitizing dye, being capable of being decomposed to producesuperacid by actinic radiation in the first wavelength range; thesecondary acid generator being capable of thermal decomposition to forma secondary acid, the thermal decomposition of the secondary acidgenerator being catalyzed by unbuffered superacid; and the image dyeundergoing a change in its absorption of radiation upon contact with thesecondary acid.